The Tomb is a natural cavern on the dried-up bed of Lake Mead, once known simply as Lake Mead Cave, that serves as the holdout of the Courier. Surrounded by landmines and other booby-traps as well as numerous scrap metal fortifications, it is accessible by a small ladder leading down to the actual cave entrance. A Note containing the Courier's eulogy can be found pinned next to the Cave entrance.
The Cave itself is no longer flooded, and its floor is filled with debris and junked PDQ Securitrons. A Securitron in the first chamber has been converted into a makeshift audio recorder by the Courier, and contains the transcript of Corier's torturing someone he believes was sent by the NCR to assassinate him. The tortured body of a Tribal can be encountered nearby, indicating that the Courier has become violently unhinged. Further within the cave is more debris and wrecked Securitons, remnants of the army activated during the Second Battle of Hoover Dam.
A single functioning Mk. II Securitron is found in the final cavern, seemingly being utilized as a guard by the Courier. It is hostile on sight and armed with its full retinue of weaponry, including Missile and Grenade launchers, making it a deadly threat to unprepared survivors. Further within the cave is the Courier himself, who is hostile as well. He will attempt to snipe the player from high ground with his unique .50 cal rifle.