Trivia: There's a small Piper airplane frozen into the ceiling of one of those caves near the summit.

Edit: According to the link JoeAltmaier pasted below, the plane has now fallen off of the ceiling. It was pretty weird to see it just hanging there, at the time.

Interview but no pictures: http://www.gorp.com/parks-guide/travel-ta-secrets-from-the-p...

amazing article. i've been up there a few times, had no idea what was under my feet

Fascinated with this type of geologic activity? Check out Lassen National Park.

Or actually, please don't -- because it's the most pristine and beautiful national park in California that isn't overrun with tourists.