Mammoth Cave is a lava tube near Tionesta, California.  I've been here about five times & have never fully explored it, though I've heard it has about 1.59 miles of passages.  The cave itself has high ceilings initially but then it becomes a continuous crawling maze of intricate passageways through wet mud & ropy pahoehoe.  This is the east end from the junction room.  All the small holes in the sand are from water droplets falling off the small lavacicles on the ceiling - each droplet carries small amounts of sand particles which as you can see has filled up a lot of the passageways.  A visit in January 2015 I got extremely disoriented deep inside & lost for about 45 minutes as I kept repeating my tracks before remembering that the entrance is higher up & the cave itself gradually slopes downwards in elevation.  From there I was able to find my way out.  This next time that I visited seven months later, there was a dead dog exactly where I had gotten lost last time, there was nothing but dog tracks everywhere & it eventually died of starvation in there.