Moaning Caverns is a limestone-marble solutional cave in Vallecito, California. It is about 200 kilometers east of San Francisco, on the foothills of the Sierra Nevada mountain range. The main show cave attraction is the 165-feet high massive hole, but the cave extends further down in smaller passages under the big room, getting to a depth of 410 feet or 120 meters. An adventure tour explores some of these parts, which is why I was here. But there are other interesting aspects of the cave as well -- at the bottom of the main room there's a large pile of animal and human bones, presumably from whoever fell down to their fate here.
Bones covered by calcite deposits:
The main room has a staircase welded together from stairs of an old warship. Scary, but regularly inspected:
The cave is also interesting in that it has both limestone and marble parts. One of the tight squeezes on the adventure tour is actually between two beautiful slabs of white and blue marble, polished smooth by previous cavers. The slabs slope upwards and are spaced just barely far enough apart that a helmet fits in. Not much room:
Other squeezy parts of the adventure route include the Birth Canal, is a tight climb out of a hole right after managing to pull yourself out from between the marble slabs. There's also the "Meat Grinder", where you need to negotiate inconveniently placed rocks, like a puzzle where you have to fit your body just the right way to pass through. It turns out that this can involve painful grinding of the sensitive parts of cavers.
There's more, chimney climbs, rope-assisted descents and ascents, and smaller detours that require climbing skills.
What I found particularly interesting was a small room opening through a hole from the Lower Mud Flat. This room is decorated by helictites. These are small cave forms, like stalactites but which changed their direction of growth in different stages of their development, for reasons that are not fully understood even today. Direction changes might be due to capillary forces, wind, bacteria, or other reasons.
Here's a picture of the helictites:
I ended up continuing my quest for 3D scans from caves. The entire cave is too big for my iPhone lidar sensors, which has only 4-5 meter range, when the big hall is 50 meters across. But I did scan two smaller rooms from the cave, the Lower Mud Flat and Helictite Rooms.
Here's a snapshot of the Helictite Room model: