This is the cave where the alien known as Nasedo lived, after he escaped from the miltary.
Most assume he pretty much directly went there from the army base, or after he secured the pods in the pod chamber. We don't know exactly how long he lived there, but usually it is thought he was there for several years.
River Dog says that for some unknown reason Nasedo became worried that men were getting close to finding him, and sometime afterward (days?, weeks?, months?), he killed James Atherton. The date of Atherton's death was Nov 16, 1959. I believe that Nasedo probably left the cave then and moved on. Most likely he wouldn't stick around if he thought he was close to being discovered.
So it is possible that Nasedo spent as much as twelve years in the cave.
As you can see from the pics, it is simply a rough, rocky cave, with dirt floors, of course Nasedo could have used his powers to 'spiff' it up a bit. We just don't know the conditions he lived in.
We do know he had the pendant necklace that he gave Atherton, and the healing stones that he left with River Dog, but we don't know if they were in the cave with him, or hidden somewhere else.
Sometime when he lived in the cave, he make the painting on the cave wall, that Michael later figured out was a map.
For complete info about the drawings on the cave wall