The Tarzana area of Los Angeles hosts a geological feature known as the Vanalden "Cave". I'd heard about it but never bothered to visit until now. Getting to it was OK, even if the "cave" wasn't.
cave
Vanalden "Cave" (
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Lava River Cave (
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After getting to 12,633' on Humphreys Peak, I descended about 20' below the Earth's surface in the nearby Lava River Cave.