Walk forward and locate the pond on the other side of the yard. Dive in, find the passage, and swim along until you can surface.

Walk around the structure to your left and enter the opening in the cliff wall - near the entrance to the chamber is a face on a column. Push it, and go through the now-open bars. Dive in the pool for Super Health at the bottom.

Return to the clearing. There's a switch on the ground near the door with three faces. Push it to open the door.