Russian Underground Tunnels.
Here is another collection of images that I think would make either a great level or at least provide inspiration for one.
These images come from Russia. To my knowledge they are not part of the normal underground system.
Any extra information about them would be appricated. If you know of similar types settings please contact me.
Metro-2 (Russian: Метро-2) is the informal name for a purported secret underground metro system which parallels the public Moscow Metro (known as Metro-1 when in comparison with Metro-2). The system was supposedly built, or at least started, during the time of Joseph Stalin and was codenamed D-6 (Д-6) by the KGB. It is supposedly still operated by the Main Directorate of Special Programmes (ru) and Ministry of Defence
The length of Metro-2 is rumored to exceed that of the public Metro. It is said to have four lines, and to lie 50–200 meters (165-660 ft) deep. It is said to connect the Kremlin with the Federal Security Service (FSB) headquarters, the government airport at Vnukovo-2, and an underground town at Ramenki, in addition to other locations of national importance.
In 1994, the leader of an urban exploration group, the Diggers of the Underground Planet, claimed to have found an entrance to this underground system.