List of minor Black Mesa locations
Black Mesa South Access Tunnel
The Black Mesa South Access is a location featured in Blue Shift in the chapter Deliverance. It appears to be an entrance to the facility. Barney Calhoun and his friends teleport here to escape from the facility.
Unnamed Decay location
This location is featured in Decay in the mission Resonance. It is a medium-sized facility near the surface level, and it contains the dampening locks. The players are sent here to reset the dampening locks to prevent another cascade from happening. There are two secondary locks upstairs, and one primary lock near a transit line. The starting area slightly resembles the test chamber from Half-Life, so it may be the upper section of that room.
Hydrofauna Studies Laboratory
The Hydrofauna Studies Laboratory is a location featured in Opposing Force in the chapter Crush Depth. It contains a series of underwater laboratories where aquatic alien creatures are kept and observed, as well as teleporters.
This location is featured in Opposing Force in the chapter "The Package", and in Decay in the bonus mission Xen Attacks. It is a large underground parking garage, and is connected to a series of train tunnels. The Black Ops assassins plant a bomb here to destroy the entire facility. The bomb is disarmed by Adrian Shephard and rearmed by the G-Man. This leads to ultimate destruction of the facility. Two Vortigaunts are also sent here earlier by the Nihilanth to retrieve crystal samples.
Level 4 Storage Unit
The Level 4 Storage Unit is a location featured in Opposing Force in the chapter Worlds Collide. It is a large basement storage area located under a parking garage. It is told the area was used by the Black Ops as a staging point until the aliens came in. The Race X also uses a sub area below to open a portal to bring in the Gene Worm.