Invergordon was an important Royal Navy base in World War I, taking advantage of the sheltered waters of the Cromarty Firth. In World War II the port served mainly as a refueling base. Large underground tanks for oil storage were constructed four miles inland at Inchindown between 1939 and 1941.
The complex comprised six tanks, linked by two parallel service tunnels. Collectively, the tanks held an impressive 32 million gallons of fuel, or around 100,000 tons. Delivery to the port was via an underground pipeline which had two pipes - for oil needed to be pumped up to the store as well as back to the port. Both the tanks and pipeline were heated to ensure the fuel retained its fluidity.
In 2009, the tunnels were specially opened for guided tours which were rapidly over-subscribed. In 2014 a world record was claimed when the (by now disused) tunnels claimed the longest reverberation record of 112 seconds after the firing of a a pistol blank.