Royal Marine's Deep Refuge - Chatham, Kent - July 2017
Built as part of the now demolished Melville Barracks in Chatham. This deep shelter was a refuge for the marines at the barracks. The history is pretty vague, but I believe the tunnels existed before the start of WW2.
In the early 1960s when the melville Barracks were demolished to make way for the council offices, and most of the tunnel entrances were sealed up.