Cave 452 is a cave located in Shoshone National Park, about a ten-minute (in-game) hike from Henry's lookout. It is the setting for several minor events in the story, and cannot be fully accessed until the player finds the trapped backpack containing the key, which Delilah's ranger friend says she lost "about three years ago."
Cave 452 is also a deadzone, and so you cannot contact Delilah while in the cave.
It is most notably the setting for the player finding Brian Goodwin's dead body, which has been rotting there for three years. Ned later claims that he died due to a climbing accident, that he didn't "sink his fucking anchor in right." The game might be making it look like a setup, possibly to influence the player to believe that Ned sabatoged Brian's gear somehow, leading to his demise. It certainly looks like his rope was cut, due to the way it was coiled up, as if it had randomly fallen.