Ho.1 was originally built as a munitions store by the Occupying German forces during WW2. This was later converted to a mushroom farm which was in operation until the end of 2006.
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This truck is British Army and was reversed into one of the branch tunnels shortly after the war, only to be sealed up and left there ever since:
High quality copies available here.
This tunnel is now under new ownership, and is an active site.