Reproduced from GREY GOLD by Samuel Murphy


Twelve years after the end of the Second World War, the `Cold War' between the Western Powers and the Soviet Union was at its height, but both parties were feeling their way towards some accommodation of their interests, an integral part of which was a limitation to the bitter nuclear arms race which was proving so costly to both sides. A test ban treaty was a major objective which both parties wanted and negotiations started in 1958, but neither side had any trust whatsoever in the other and the talks proved difficult.

An essential pre-requisite of such a treaty was an ability to make sure that none of the signatories could cheat by carrying out secret tests undetected, and it was arguments about the effectiveness of the systems for detecting surreptitious nuclear tests whether they were in the air, on land, in the sea or underground which deadlocked the talks. Then out of the blue came a potentially devastating bombshell in the form of a new scientific theory. An American scientist, Dr A.L. Latter, propounded the theory that it should be possible to camouflage the seismic signal from the detonation of an underground explosion so that it could either be missed altogether or made to appear many times smaller than it actually was. If this was true then the impending treaty was in jeopardy, for without mutual trust and without cast-iron guarantees that a transgressor could be detected, signing an agreement was just too dangerous for it could allow an unscrupulous state to obtain a decisive technological advantage over the others. No-one had any idea as to whether this new theory, which became known as the Latter Decoupling Theory would actually work as predicted, but it was essential that it should be tested before the treaty was concluded. On Government orders, scientists in the USA and in the UK set to work to devise experiments to check it out.

When an explosive charge of any type is detonated underground, the vibration set up by the blast is transmitted through the rock and over the surface of the land, and if sensitive equipment such as a seismograph is set up, tens, hundreds or even thousands of miles away, the minute effects can be recorded and the size of the blast estimated. Latter's idea was that if the explosive charge was set off in the middle of an open chamber, of the same size as the region of broken rock which would have been formed if the explosive had been tightly packed into a small hole in the rock, then the pressure wave even a short distance from the seat of the explosion could be attenuated by a factor of about a hundred or so. To get the optimum effect the chamber had to be sized according to the amount of the explosive and the depth below ground, so that the pressure of the explosion was about equal to the pressure due to the weight of rock overhead, and this meant that a smaller chamber was needed for a deeper hole. The 'decoupling' of the explosion from its tell-tale seismic signal had to be tested by comparing normal or `coupled' tests with 'decoupled' tests to see if there really was a difference, and this involved calculating the sizes of chambers needed to produce the effect from Latter's theoretical work. Since no one knew what the size of the effect would be in practice, guesses had to be made and applied to small scale tests, the data from which could then be used to plan bigger tests.

A race to carry out the necessary tests started between scientists in the USA and the UK, and in England a team of geophysicists at the UK Atomic Weapons Research Establishment (AWRE) at Foulness was asked if a simple test could be carried out quickly. Dr Eric Carpenter, a young geophysicist at Foulness, said that they could, and immediately tried a small experiment with two charges of explosives in a 20 feet deep bore hole in a bed of clay. The first 65 lb charge formed a large hole, and the second 2 lb charge suspended in the hole was 'decoupled' according to the Latter theory. It worked - the predicted attenuation was obtained! This success provided the impetus for larger scale tests, and a group from the AWRE (Aldermaston) was put together which included the scientists from Foulness. It was headed by Dr Iuean Maddock, Head of the Field Experiments Branch of the AWRE, who was appointed Project Superintendent in charge of the tests. Part way through the programme, Dr Thirlaway, who was then working for the UN in Pakistan as a seismologist, was invited to lead the team which later developed into a world famous group of seismologists based at Blacknest, near Aldermaston.

The UK test programme, which was to carry out a series of tests of different sizes, was codenamed Operation Orpheus, and was designed in conjunction with a different programme to be carried out by the Americans codenamed Operation Cowboy. The Officer in Charge (Operations) was R.E. Drake-Seager from AWRE Aldermaston, and the Officer in Charge (Measurements) was J.K. Wright of AWRE Foulness. Under Mr. Maddock, a search was made for suitable sites for the tests. An abandoned Cornish mine at Callington was soon found where small charges would be tested, but large scale tests needed a suitably deep mine which was either abandoned but in good condition, or was nearing the end of its working life. Greenside Mine, which fitted the latter description, was finally identified as the most suitable location for the large scale experiment.

Operation Orpheus actually comprised three principal phases, of which the Greenside work was one:

  • Phase A: Small charges fired in a 6ft diameter cavity in granite and shale at depths of 100 to 300 feet in the Excelsior Tunnel in Callington, Cornwall.
  • Phase B: Participation in the Operation Cowboy tests, where charges of 3,000 lb would be detonated in 30 ft diameter cavities at depths of 800 ft in a salt mine in Louisiana, in the USA.
  • Phase C: The Greenside tests, where a 3,000 lb decoupled test and a 1,100 lb coupled test would be carried out in andesite rock at a depth below surface of 1,700 ft and the results compared. The charge sizes were chosen so that if the decoupling worked as predicted, the seismic signals would be similar in intensity.

Terms were quickly agreed for the handing over of the mine to the AWRE for the short testing programme of Phase C, and the announcement that the mine had been selected for seismic tests was made on Wednesday 25th November 1959. Great care was taken to quell any unease amongst the locals by emphasising that no fissionable material whatsoever would be involved in these tests.

The Greenside Company put the mine entirely in the hands of the United Kingdom Atomic Energy Authority (UKAEA), but the Company contracted to carry out all the civil engineering and mining works associated with the programme. Thus three different groups worked together: the Greenside Company, who provided the labour, tools and equipment, the Operations Group of the AWRE who planned the excavation work, obtained the explosives and other materials and directed the work, and the Measurements Group of the AWRE who installed the seismographs, collected the measurements and carried out the calculations. About fifty Government personnel were involved, and in November and December they arrived in Glenridding. Most stayed at the Glenridding Hotel, others were billeted at the Traveller's Rest nearer the mine.

The mining work was directed by a large body of UKAEA personnel who were in the main quite ignorant about mines or mining, and there was some concern that they might blunder into dangerous situations which would be accepted as normal and avoided instinctively by experienced miners. To preserve these men intact for the duration of the work, basic rules of conduct, which explained the hazards to be expected when working in a mine, were drawn up and laid out clearly in a Safety Guide. A police check point was set up at the Lucy Adit mouth, and no one was allowed in until they had signed a declaration that they had read and understood the Safety Guide. This check point also issued tallies which showed who had entered and left the mine, and the police officer on duty made sure that no matches or other contraband were taken into the mine. Many of the old Greenside men were offended by these regulations and in particular did not take kindly to being barred from their 'own' mine.

The position chosen for the tests was the 175 fathom level, where Cyril Connor's exploratory drive, the West Crosscut, had left the mineralised and fractured vein and penetrated deep into the solid rock well to the south of the Clay Vein. This crosscut terminated in a solid wall, and originally it was intended to drive a short crosscut of minimum size at right angles to it and excavate a chamber at the end. Unfortunately, news came during the planning phase that a technical discussion was to be convened in Geneva for November 1959 and political pressure was applied to speed up the work, so a rather cruder experiment than originally planned was carried out.

Basically this meant that there was no time to drive the small crosscut, and instead the chamber was made by opening out the remote end of the West Crosscut. The Greenside miners set to work and shot out a roughly ellipsoidal chamber with a maximum cross-sectional diameter of 16 feet and a length of 25 feet. The volume of the chamber was 2,850 ft, so that the steady-state pressure of 2,000 psi generated by the explosion would be approximately equal to the pressure of the rock at that depth. The coupled charge had to be as close as possible to the main charge so that an accurate comparison could be made between the two, yet far enough away to prevent a sympathetic detonation of the explosives by the shock wave from the first explosion. To get good coupling the explosive had to be in good contact with the rock, so a narrow little adit was driven at right angles to the West Crosscut at a distance of 75 feet back from the big chamber. This adit was 4 feet by 5 feet in cross-section for a distance of 15 feet and then tapered to a cross-section of about 2 feet by 2 feet for a distance of 5 feet.

As the mining work was being completed, the explosives which comprised a total of 4,170 lb of RDX TNT in 12 lb packs, were brought up to the mine in a military powder wagon under heavy police guard. At the Lucy Level entrance the explosives, still in their wooden transit cases, were taken from the powder wagon and loaded onto the mine wagons which were ready coupled onto the Lucy loco, waiting at the adit mouth. Each wagon carried 240 lb of RDX TNT, in four wooden boxes. Safety precautions were rigidly enforced: no smoking was permitted within twenty yards of the powder wagon and the vehicle remained under police surveillance from the time it arrived until it had been fully unloaded. The Lucy loco pulled its delicate cargo at a sedate 4 to 5 mph along the 1,300 yards of track to the top of Smith's Shaft, where each wagon was detached from the train, put singly into the cage and lowered to the 90 fm level. Here the process was repeated, each wagon was pushed the 200 feet from the bottom of Smith's Shaft to the top of Murray's Shaft and lowered down the incline to the 175 fm level. On arrival the trucks were again propelled by hand about 700 yards southwards along the 175 fm level and the West Crosscut, where they were unloaded, the charges removed from the boxes, and the empties returned. While the explosives were being carried down the mine, all normal activity was suppressed to minimise the risk of an accident.

The big charge was put into the chamber first and some rather surprised seismologists who had come to the chamber once their work was done to while away an hour or so watching a photographic session, were cheerfully volunteered by their boss, Dr Eric Carpenter, to help load the packs of TNT onto the wooden platform which occupied the centre of the chamber. Peter Marshall and Ron Burch were amongst this group and they vividly recollect the work, lifting and stacking the packs of explosives, while a group of photographers, electronic flash units popping merrily, recorded their nervous endeavours.. The 3,010 lb charge consisted of 7 layers of 36 boxes, each box weighing 12 lb. Multipoint detonation was used, employing 36 detonators embedded amongst the TNT and connected by wires to the firing circuits. A smaller charge of 1,160 lb was packed firmly into the small crosscut, then the detonators were put in place and connected into the cables of the firing system.

Once both charges were in place the West Crosscut was sealed by a heavy stemming. The change to adapting the main crosscut level for a chamber meant that the 6 ft wide level had to be filled rather than the small tunnel originally envisaged, and this posed a major problem. Normally a concrete plug would have been used, but this could not be constructed in the short time available. On the advice of Major Kerr, the stemming was made up of sandbags filled with tailings from the mine dam, interspaced with gaps and timber walls. The main stemming for the decoupled charge filled the level back beyond the position of the coupled charge so that it formed an integral part of both. Sand was again used for the stemming of the coupled charge, the crosscut being filled, then the West Crosscut stemmed for a distance of 15 ft each way from the little crosscut.

Two of the AWRE men assembling the decoupled charge on a wooden platform in the middle of the egg-shaped chamber at the end of the West Crosscut. Dr. Eric Carpenter is lifting a pack of TNT from its box and Ron Burch is on the ladder behind him.

Both charges and the stemming were put in place by 17th December 1959, when the West Crosscut was sealed by two brick walls, one in the crosscut and one in the nearby South Drive.

The equipment which would fire the charges was installed in the West Crosscut at a point 285 ft from the smaller coupled charge, and was controlled from the surface.

Meanwhile the seismologists were making their own preparations. There was some difficulty at first in finding enough equipment for the Cornish (Excelsior Tunnel) and Cumbrian tests, but four sets of shortperiod Willmore seismometers were ordered, each of which could simultaneously measure vibrations vertically and in two horizontal directions, and with other equipment already available, enough equipment for six measurement sites was collected and sent to Greenside.

An enormous amount of cabling was also needed for the detonation and safety systems associated with the firing of the charges and this was brought on a trailer pulled by a huge and immensely powerful military transporter called a Matador. The Matador could haul its cargo of cable drums and machinery at a maximum speed of 15 mph along the winding roads to the test site. Ron Burch, one of the Foulness seismologists, accompanied the driver of this vehicle on its journey, and he recollected that on the way back they overtook Dr Barbara Moore, an anti-nuclear demonstrator of the time, on one of her epic marches for peace. This lady covered ground at a formidable rate, and the driver reckoned that it was a close thing as to who would overtake who! This vehicle was parked up at the mine beside the old lodging shop, now deserted, after unloading its cargo. The miles of cable which connected the control and firing station, set up in a wheeled trailer besides the Office, with the equipment in the mine, were laid along the Lucy Level then threaded down the two shafts to the 175 fathom level and along to the firing station in the West Crosscut by the time the explosives had been put in place.

Six sites were selected for the seismometers at distances varying from half a mile to 47.5 miles, distances which were chosen to determine the limits at which the signals could be detected. Seismometers are almost unimaginably sensitive, for they are meant to record tiny vibrations from distant seismic disturbances. For example the detonation of a fully-coupled one kiloton nuclear device in what was then the Soviet Union would produce a vibrational movement of the ground of just one nanometre in the UK, a dimension which is about one fiftieth of the diameter of a flu virus!

Almost anything can produce vibrations of that size, so sites were chosen where there were no busy roads, trees through which the wind could whistle, or nearby becks which could mask the weak signals from the Greenside explosions with their own background noise. A nearby telephone link was essential in case radio communications failed at the critical time, for all the stations had to start recording just a few seconds before the detonation, and the time signal 'pips' on the BBC radio transmission or the GPO talking clock were used to obtain this precise timing. The sites which were selected for seismic recording were: 'Foulness Castle' NGR 364175, a code name invented by Eric Carpenter for the Pitchfords' house near the Low Mill, 0.5 miles from the test point, where the nearest seismometer was located; Low Hartsop NGR 412129, 4.5 miles from the mine at the end of the road running through the village; Outgang Farm, Helton, near Penrith NGR 506219, 9 miles from the mine in an old quarry near the lane leading up from Helton village; Dry Howe Farm, Selside, in Sleddale NGR 528021, 14 miles from the mine in a field to the south of the farm buildings in a fairly well wooded valley; Underwinder Farm, Sedbergh NGR 643927, 23 miles from the mine at a height of 600 feet up the side of a hill; Malham Village, Yorkshire NGR 903631, a site 47.5 miles from the mine in a field by the Malham to Malham Tam road on a limestone outcrop 300 yards from the village.

During installation and testing of the seismometers, several of these very delicate instruments broke down, so this farthest site was abandoned and the instruments moved to the other sites to replace the failed ones. At Fowlness Castle two instruments were set behind a rough wall of stones and turves, and protected from the elements by a plastic sheet. The data from the instruments were transmitted through cables to the recorder set up inside the Pitchford's cottage.

The intention was to detonate the large decoupled charge first, then thirty minutes later on the next time signal, detonate the second fully-coupled charge. Before the first firing, an attempt was made to estimate the strength of the seismic signals which would be obtained from the tests by firing a small coupled charge in a 10 ft long hole in the 175 fathom level. This was a charge of 100 lb of Polar Ammon dynamite 'borrowed' from the Greenside Co. and it was detonated at 19.00 on 15th December 1959. Only the Foulness Castle station detected a signal from this explosion, so the scientists altered the amplification of the detectors to increase sensitivity.

The signal from the Foulness Castle Geophone detector recording the shock wave from the decoupled explosion

Now all was ready for the big event, the detonation of the 3,010 lb decoupled charge on which the future nuclear policy of the country depended. On Saturday 19th December the mine firing circuits were connected to the firing cables running through the stemming to the charge in the big chamber and the special interlock keys without which the firing equipment was inoperative were inserted. The mine was cleared of personnel, the entrance sealed, and the count-down commenced. At each of the five detection positions, two scientists crouched in the shallow holes or small enclosures which contained the seismometers, in radio and telephone contact with the firing trailer, ready to start their equipment on the radio time signal. A night firing was chosen so that the background noise from human activity would be at a low level and as the moment approached Harold Nobbs in the firing trailer fitted the Master Safety Key into the control panel, thus making contact with the slave control unit on the 175 fathom level and on the radio time signal for 22.30 the button was pressed.

On the surface there was a dull thudding noise and the remote seismograph recorders fluttered briefly as they recorded the great event; it was all over in less than a second. The second charge was then connected to the firing circuit but to everyone's great disappointment it was found that there was no electrical connection to the slave control system which had been destroyed by the failure of the stemming. The firing of the coupled charge would now have to wait until the damage was repaired, so the AWRE men of the Measurement Group packed up and went home for Christmas. Ominously, the Sedbergh seismometer twenty three miles from the mine had been unable to detect the large decoupled explosion, giving an early indication that the strength of the signal had been much weaker than predicted and thus that decoupling might be even more effective than they had estimated for large explosions. This result was communicated to the negotiators at the Geneva talks, and the significance was not lost on them.

There followed a period of waiting as the mine was cleared of noxious gases from the huge explosion. It had been calculated that the first charge would release over 20,000 ft of poisonous carbon monoxide, small amounts of hydrogen cyanide gas and nitric oxides and nearly 5,000 ft of explosive hydrogen gas, so the test area had been sealed off with brick walls and a large Roots blower installed to pump out the dangerous gases through a pipe into the shaft, fresh air coming in through a one-way valve in the wall to replace the exhausted gas. The blower was switched on before the detonation and although some of the containment walls were destroyed, the blower was undamaged and it was able to carry on clearing the air after the detonation. Re-entry of the mine was a dangerous process and this was carried out by trained rescue teams from Greenside and neighbouring mines who were used to wearing breathing apparatus; the 'greenhorn' AWRE men were banned from the mine until it was declared safe.

A PROTO closed circuit oxygen apparatus had been in standard use at Greenside since the 1952 disaster and this equipment was used by the men who worked their way down into the mine, checking it section by section under the supervision of the Senior District Inspector of Mines who was familiar with the workings. These men were equipped with portable gas detectors, but also relied on an older and well proved technology, for each man carried two canaries in cages, and had instructions to proceed with great care if the birds should show any signs of distress. As Officer-in-Charge, Mr Drake-Seager was allowed in during this work. Cages of mice had also been left at strategic positions in the mine before the charge was fired, so that if they were found alive afterwards this would reassure the re-entry teams that no poisonous gas had reached that point in the mine. Moving cautiously, fresh air bases were progressively advanced in the mine by the masked re-entry teams, until finally the Inspector informed Drake-Seager that normal entry without breathing apparatus was permissible.

The Operations group then was allowed in and work commenced on repairing the damage to the stemming and control equipment so that the second charge could be fired. When the test site was examined, it was found that the stemming in the West Crosscut had failed to contain the blast and had been blown along the length of the level. The slave control equipment and firing cables for the second charge had been shot down into the far end of the West Crosscut and completely destroyed. The explosive charge and the detonators were still in place in the second crosscut and were usable, so cables were connected to them, then the stemming was replaced and the firing system restored. The work was completed early in January 1960, and the AWRE Measurements team returned for the second firing which was scheduled for 15th January.

During the night of 14th January 1960, the day before that scheduled for the firing of the coupled charge, a group of Greenside men were finishing their work in the West Crosscut level by sand-bagging the ventilation pipes under the supervision of the afternoon shift boss, John Pattinson Brown, known by his nickname 'Jackets'. Among them were William Sinkinson, who was twenty-three years old and a middle-aged man called Alex Santamara. These men were normally surface workers, and although Sinkinson had worked underground occasionally, neither were experienced miners. After the mid-evening break at about 9.00 pm the younger man took Santamara off to visit some of the stopes in the northern part of the mine which he had worked in some years before, where he said good pieces of white spar could be had for a garden decoration. Arnold Lewis was another of the miners and at about 10.30 pm he suddenly realised that the two men had not returned to work and told the shift boss. Brown called Fred Dawes over, for he thought the only place they could be was in a stope which Dawes used to work and the two men set off to look for them. There was no sign of the men in the 175 fathom level, so when they reached Dawes's Stope, Fred Dawes clambered up the 20 ft high waygate ladder. When only three steps up the ladder he realised that the air was tainted but he continued up with the smell getting stronger until he reached the top and peering into the gloom saw the motionless body of Alex Santamara lying on the lowest bench.

The two men went back to get the others and when they all arrived back at Dawes's Stope, Brown went to get a rope while Arnold Lewis and Fred Dawes climbed the waygate ladder and entered the stope. Lewis had been involved with the 1952 rescue attempt and recognized the presence of gas, but filling their lungs with air the two men dashed along the bench, seized the inert body of Santamara and hauled him back to the entry winze. Gasping and dizzy, they realised that they had been affected by gas and couldn't get the man down the ladder way, so they left him propped up close to the ladder where the air was freshest, and went down to get help. The returning shift boss met them as they got down into the level and Arnold Lewis just had time to warn him of the gas before passing out. Brown then telephoned Robert Corlett to call for a doctor and alert the rescue team and the alarm was raised.

Fortunately, since the next test was imminent, a fully equipped rescue party could be called out immediately under the control of James Barnsley, one of the Greenside hoist men. A small group of men, fully-equipped with breathing apparatus was sent in to bring out the two men from the stope and take their would-be rescuers to safety. Santamara was quickly brought down into the 175 fathom level and even though there was no sign of life he was given artificial respiration until Dr Armstrong the village doctor arrived and confirmed that he was dead. Barnsley and his men went back and found Sinkinson up in the stope wedged in a hole near the top of another ladder and he too showed no signs of life. The rescuers were able to move the body so that Barnsley could get past to the top of the ladder and then lower it to the bottom bench, but then they all had to retire from the gas-filled area and. go back to the surface to change the breathing equipment. The tired men were on the point of setting off once more when a fresh rescue team from Kirkby Thore arrived and Barnsley led them back to the stope to recover Sinkinson. Lewis and Dawes recovered from the effects of the gas, but Santamara and Sinkinson had probably been long dead when they were first found.

News of fatalities in the mine was broadcast over the radio, but the AWRE immediately telephoned the families of each of their men who were working at Greenside to reassure them that their menfolk were not involved, a thoughtful touch for which the men were deeply grateful. At the inquest no blame was attached to the scientists who had failed to detect gas in the old stopes, for the men had violated the Greenside Mine's own Safety Code which expressly forbade entry into abandoned workings except in the company of a responsible mine official. However the plucky rescue attempt by Dawes and Lewis was universally praised and in the November of that same year, the two were rewarded for their heroic efforts on behalf of their fellow miners by the award of the Queen's Commendation for Brave Conduct.

The shocking deaths of the two men put a stop to all further work on Operation Orpheus for several weeks while the situation was reassessed. The Inspector of Mines had other commitments which prevented any further involvement with the project, so the AWRE Operations Group, their attention focused on the hazards of gas by the accident, sought the advice of a consultant, Professor Graham, who was an expert on gas and from a Mr. Carson of the National Coal Board who for some years had been involved with rescue and re-entry work in mines in the Northumberland area. On their advice the containment walls were strengthened and gas extraction improved by the installment of a new Roots blower with an extraction capacity four times greater than the previous blower.

Two seismographic detectors, a little Geophone and a large Willmore set on a handy rock in a shallow enclosure behind the Pitchford's house.

With a higher pressure extraction system, the danger of a hydrogen explosion in the venting gas was considered worthy of attention and an air bleed system was fitted into the vent pipes so that the gases being dragged out of the explosion area could be diluted with fresh air, thus keeping the hydrogen concentration well below the critical level. The scientists changed their minds several times as to just where the holes should be in the pipe which ran the length of Smith's Shaft, and Harold Oglethorpe who was doing the work for them, his patience wearing just a trifle thin, asked them quietly whether it would perhaps better suit their requirements if he were to bore holes at one foot intervals the length of the shaft so that they could choose which one to use at their leisure? This gentle irony was taken in good humour by the scientists.

Over three months after the accident, on Friday 29th April 1960, the mine was once again cleared of personnel and the countdown commenced for the detonation of the fully-coupled 1,160 lb charge of RDX TNT. This time all went smoothly and according to plan; at precisely 19.00 the button was pressed and the seismometers at all sites recorded the shock. Background noise was very much lower than it had been before Christmas and clear traces were obtained. The venting of the mine and checking for gas was carried out a great deal more thoroughly after this second test, so it was some time before the mine was declared safe once more.

Despite the trouble with some of the seismometers and all the unknowns in the settings. to be used, the Greenside tests were a resounding success, giving the UK scientists a world lead in this strategically important area. Analysis of the results was conclusive: the signal from the smaller coupled charge was much bigger than that from the large decoupled charge, suggesting a decoupling attenuation of the signal by a factor of between 10 and 30. The Latter theory worked for large explosions! Talks continued with the Russians for a while but the problem of verification was insoluble and in August 1961 the Soviet Union resumed its nuclear testing programme; the test ban treaty was in the dustbin.

In 1963 efforts were renewed and a partial treaty was concluded which banned nuclear tests in space, in the atmosphere and underwater. Underground tests were still and even today, remain a problem, for in the intervening period decoupling methods have been improved to give attenuation factors of 70 or more for nuclear explosions, so that even with the more sensitive detection systems now available, a decoupled explosion would be hard to detect with a seismometer. Satellite surveillance systems incorporating remote sensing devices are now so highly developed that even underground explosions are difficult to hide, but decoupling is still a problem: very small nuclear explosions can be effectively decoupled and made totally invisible from the air.


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