A friend of mine was into extreme caving (i.e. more often than not crawling into deep places you're not really meant to be crawling into) and did an exploration of a military installation on the Mendips near Bath (UK). Strictly speaking it's still classified, but obviously deserted for decades.

One of the interesting finds was the diesel generator for the installation (which consists of miles of tunnels and caverns). It was missing a head and my friend was able to climb completely into one of the cylinders - that's a big generator! One of the other interesting finds was the fuel tank for the generator, which was still half full of red diesel (red dye is used to indicate that tax hasn't been paid - for military or agricultural use only). There must be hundreds if not thousands of gallons of the stuff in there

Rob