In 1941, the Germans began building tunnels for protection of crew and material on the Channel Islands of Jersey and Guernsey. In Jersey alone, there were plans for up to 25 tunnel systems. Hohlganganlage 4 on Jersey was one of them and was to be used for storage of ammunition.
The Germans used slave workers for the hard work of chopping through the underground, a both degrading and very dangerous task.
Ho4 was never completed, but was still put into used before the Germans capitulated in May 1945.
For a short period after the war, the English military used the depot, but short after and until today it is used as warehouses for Jersey waterworks.
However, due to poor ventilation, the entire tunnel system is not used.