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Kill the headcrabs in the room, and then exit the room via the other side, avoiding the barnacles.
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It would not be a bad idea to stock up on ammo at this point - it will regenerate after a few seconds, so don't be in a hurry to leave. Kill the aliens and then cross the room, using the grapple to attach to the alien plants.You'll need the grapple to cross this room, but it will be fairly hard with three aliens spitting acid at you. You'll be confronted with a greenish room with alient ammo growing all over the walls.There's another doorway in the habitat floor - shoot out the glass, work your way around the ledge of the habitat, drop down and head through the doorway. When you get off the elevator, make your way past some rubble, through a doorway, and around past some aliens overlooking another alien habitat.One path leads to an elevator going down, the other leads to a door somewhat blocked by a yellow construction sign. Make your way around to a fork in the hallway.Jump on the door and enter the upper hallway. There's another elevator opening with a broken door providing a small platform to jump on. At the top of the elevator shaft, drop to the small ledge and look down.Climb out onto the roof and then up the ladder in the elevator shaft. Break the glass and climb in the elevator.
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Go around the hallway and make your way to an elevator shaft.There are two doors exiting this room - one is the previously locked door heading back to the observation area.
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