Pluto's Cave is a lava tube north of Mt. Shasta, California, about 1/2-3/4 miles in length.  It is 190,000 years in age & was discovered in 1863, however prehistoric use of the cave is evident.  John Muir visited the cave sometime during 1874-1875 & in the early 20th century it was used by the Improved Order of Red Men (as indicated by their graffiti).  There are a few collapse pits & this skylight, which is perhaps the prettiest part of the cave as a beam of winter sunlight passes through the suspended dust into the darkness.  Like other caves in the area, graffiti & vandalism has taken their toll at this locality.  The surrounding area has homes that are packed w/ marijuana farms (now that it's legal here).

More info here: 

https://en.wikipedia.org/wiki/Pluto%27s_Cave