Been watching the clips by this Russian guy that call himself MSh: he sneaks into all kinds of abandoned and secret places and tapes it. Everything is in Russian, but here are some clips:

Object 1180: Underground Strategic Command Bunker


Object 221: Reserve Command Bunker of the Black Sea Fleet


Secret Undeground Laboratory


Stalin's High-Rise Building


ZIL Factory Stamping Workshop


Vehicle Bunker of Bulat Factory