Stairs lead down to the base of the canyon you overlook but first kill the Mercenaries guarding the stairs and a couple more below.
When they are all dead, investigate the two huts. There are flies in both and a Tentacle in one, but they contain valuable ammo, flares and a force field. Proceed down the canyon to a temple. If you approach by the drawbridge, it will automatically lower. If you jump into the moat and swim to the extreme left-hand side, you’ll find a small cache dug into the wall of the temple filled with ammunition and supplies.
Cross the bridge, fight the Mantas, and enter the temple.