Subnautica

This achievement is for finding the alien thermal plant in the inactive lava zone. The thermal plant is inside a structure referred to as the "lava castle."

Coordinates
[-75 -1182 5]

The inactive lava zone is located at the end of the lost river past the giant cove tree. As you go through this eventually you'll come to a large open area. There are two sea dragon leviathans that swim around in this area. In the middle of this large open area is where the lava castle is located. The entrances to the castle can be found by the lava that flows down the structure. I also have made a video guide for the game with all of the achievements here.