Fallout: New Vegas locations project
The sub-basement has a simple layout and is largely unfinished. The player is deposited into a small area below the casino with an explosive crate. Blasted through the walls and the living rock is a passage into the technical sections of Vault 21, sealed from the world by Mr House. The tunnels lead under the outer Strip wall into the wasteland, making it a perfect way to get on and off the Strip unnoticed - and it doubles as an emergency escape route.
The sub-basement is not accessible under normal circumstances. However, the player can trick the game into unlocking the elevator by triggering Benny's flight (any method works, though talking him into going to the Presidential suite and forgiving him works best) and killing him or reaching the elevator before he does.
The sub-basement appears only in Fallout: New Vegas.