There's some wineries in the area and the little town that the Cavern is in is (called Murphys) is quite delightful, w/an old-school, well-kept downtown, lots of little cafes with wine-tasting of the local product. Great day trip from the Bay Area up there ... pretty sure you could do both Moaning and Mercer in a single day, with lunch in Murphy's in between.
https://www.tripadvisor.com/Attraction_Review-g32763-d266781-Reviews-Mercer_Caverns-Murphys_California.html
There's literally like 30 distinct/different 'formations' they show you on the tour, it's really magnificent down there. You do descend I think like 16 stories or something like that (but tons of stops to look at stuff) ... so be prepared for a bit of a hike. If you like caves and esp. large and unique formations (I think a handful they've got exist only at Mercer so far as it's known), this is one o' the best, and I've been to lots.
Also makes for a great stop on the way up to Yosemite via the 108 entrance. And I tell you what ... scenery doesn't GET a whole lot more breathtaking than the descent into the Valley along the 108 route. It's flippin' amaze-balls.
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