Bennet, Simmons, Rosenberg and Calhoun about to leave for good.
After a guarded gate and a 5-mile road tunnel named "Black Mesa South Access Tunnel", it leads inside the facility, near the Freight Yard. There the end of tunnel is named "South Exit".
Its inside end is seen at the start of the Blue Shift chapter Captive Freight, and its outside end at the very end of the last chapter, Deliverance.
When reaching the Freight Yard, Barney Calhoun notices that the switch of the South Exit door has been sabotaged by the HECU, preventing anyone from going through.
Later, after Barney opens the portal in Section A-17 Prototype Labs for Rosenberg, Walter Bennet and Simmons, sending them to the outside end of the tunnel, Rosenberg and Simmons prepare a Black Mesa SUV they will use to leave the compound, while Walter breaks the gate open with a crowbar. When Barney's turn comes however, he is caught in "some sort of resonance displacement", and is unable to reach the access tunnel right away, being first sent again to Xen, then in a small store room where he witnesses an unconscious Gordon Freeman being dragged by two HECU soldiers in an industrial area of Sector E. After he finally reaches the scientist, they leave the compound forever, to an uncertain future.