The Alternate Entrance 1 is an entrance in survive and kill the killers in area 51. Before the killer mode update, it was the only other entrance. It also has a Secret Entrance located in the top right corner of the room, above a potted plant.
It's good to use this entrance in Killer Mode, as most of the killers camp at the main entrance and farm players. The ammo box can be found, and on some occasions, a code can be found on the wall behind the computer. If someone closes the door to get into the Alternate Entrance 1, there is a button to open it underneath the desk.