Watch on HDtoday. This documentary follows film photographers Simon Riddell and David Allen as they try to turn an underground WWII oil storage facility in the Scottish Highlands into what may be the world’s largest darkroom. The once top-secret Inchindown storage facility comprises six fuel storage tanks and two access tunnels. When the facility was built in 1938 it was the largest underground man-built structure. Each tank measures 237 meters long, 9 meters wide, and 13.5 meters high.