Excelsior Tunnel: UK Atomic Weapons Research establishment chamber (Public)
Kelvin Lake beside one of the AWRE chambers. The Excelsior Tunnel was the site of Phase A of Operation Orpheus and involved a small charge being fired in a 6 foot diameter cavity in granite and shale at depths of 100 to 300 feet. Phase B took place in the US.
Moderator Edit, the 3rd part of the de-coupled tests that proved the theory correct took place in Greenside Lead Mine, Ullswater, Cumbria.
The test took place deep underground and proved the theory to be true.
More info here
(Photo of Excelsior Tunnel uploaded by (user 1633 not found). Please do not copy without permission).