Lava tube cave in the Lava Beds National Monument (California).

This one was called "Catacombs", and there was a sign indicating that it was closed due to "possibly unsafe conditions". I think that they are just afraid that visitors could get lost, since this lava tube has a maze of branches and is rather large.

For more info about lava tubes, read en.wikipedia.org/wiki/Lava_tube.

For more info about this place, read en.wikipedia.org/wiki/Lava_Beds_National_Monument.