Sometime after the destruction of the Oil Rig, the New California Republic sacked Navarro, spreading the Enclave across the wasteland. Some members of the Enclave took refuge in the bunker, while others integrated into other towns and settlements. According to Judah Kreger, the bunker is an old Enclave Vertibird refueling station.
The bunker is sealed by a password that can only be obtained through Arcade Gannon's companion quest, For Auld Lang Syne. After speaking to these five members of the Enclave remnants, you will have collected the password necessary to enter with the password "Dear old friends, Remember Navarro".
The layout of the bunker is similar to the Hidden Valley bunker, itself modeled on the interior of Raven Rock. The design is very linear. The entrance leads to a staircase, which opens into a small hangar. There is an Enclave VB-02 Vertibird, six suits of Enclave power armor, each individually protected by forcefields (and thus impossible to take), several armor and weapons lockers by the power armor (only one of which has loot in it), and a pair of crates near the back. All of the items are considered owned, but there is no Karma penalty for stealing them. Beyond the hangar is a small planning room, where the Remnants will have gathered to decide which faction they should aid in the upcoming battle. About fifteen or twenty feet north of the entrance is a hollowed out rock with random loot.