The “Hydra” station, and much of the facility used by “Others” early in the third season (“A Tale of Two Cities,” Ep. 3×01) is the former Paradise Park (and its four-story aviary), located deep in Manoa Valley. The facility still houses Treetops Restaurant, but the lower and ground levels are not open to the public. This location is near Manoa Falls Trail.
Map: 21.332, -157.801