After World War II, Sweden took the decision to build a protected power station deep underground. This would ensure the continuity of power in the event of another conflict. Key requirements were at least 50 metres of rock covering above the underground generator halls, year-round access to fresh water for cooling purposes and access to ice-free harbours for delivery of fuel.
A list of sites was evaluated and the chosen site was on the west coast at Stenungsund - the town name means the sound of the village at the foot of the mountains. Construction started in 1955 and involved a workforce of around 500. Two tunnels over 2.5km long four enormous generator halls were the first to be excavated. The first of four power plants came onine in 1959 and the fourth in 1969.
A total power output of 880 MW made Stenungsund the largest oil-fired underground power station in Europe (some hydro-electric stations are larger). A total of 7 kilometres of tunnels linked the production facilities and the exhaust exited via four huge chimneys. As the chimneys potentially provided a useful aiming point for enemy forces they were built with voids inside the mountain so that a collapsed chimnet would not block the exhaust pipework.
The plant went onto a standby basis in 1998 and supplied power only during periods of exceptional need. The last time the plant was run was in 2015.