The cave passage is a small cave system in the Anchorage Reclamation simulation in 2277.
The passage has nothing really of note. It is basically an alternate route of getting to the Chinese artillery outpost without having cross the bridge there. This could be a safer alternative since the bridge is often under extreme fire from the Chinese soldiers in the bunkers near the artillery outpost. The passage is accessed through a door in a passageway near the bridge and leads straight to a door that opens to a vantage point on the bunkers.
The cave passage appears in the Fallout 3 add-on Operation: Anchorage.