Distance from SF: 140 miles; 2.5 hours
At 410ft, this is the tallest public cave you can visit in California. Its name comes from a moaning sound created by drops of water that fall into a bottle-shaped flowstone formation. The bones of 100 people from as far back as 13,000 years ago were found at the bottom of the cave. It's believed that these unlucky folks accidentally fell to their deaths throughout the ages. Tours of Moaning Cavern start with an optional rappel down to the cave floor.
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Alexandra Kenin is the founder of Urban Hiker SF. She left the tech scene two years ago to explore the stairways, hills, and hiking trails of the City by the Bay. You can find her on Twitter, Facebook, and Instagram.