TSIL436 = The Secret in Locker 436
This is a good read: Mole People Are Known To Inhabit These 15 Places Around The World.
My favorite is #3, a Cold War relic:
Hidden beneath the ground someplace in the UK, the underground city of Burlington is one of the most secretive cities in the world. Built to house 4,000 government officials, it also had a BBC studio, 60 miles of roads, a railway station, a hospital, water treatment facilities, and a pub. It now sits derelict since the end of the Cold War in 1991.