I’ve seen lined manmade lakes before. No idea why they line them, probably wanting to keep the water out of the land, maybe just to not “lose” it, maybe pollution related. Now why that far out in nowhere somebody made a lake? Dunno, but it’s pretty “typical” for such a project.
One identical filled site just below the open area. Since it’s part of Naval Air Weapons Station (NAWS), it’s likely some wastewater dumping pit. The surrounding area with tanks and military vehicles looks like ordinance testing.