Robber Barron Preserve

A good chance to see San Antonio’s underground scene!
This is a giant sinkhole that has been turned into a little park around the entrance to Robber Barron Cave. Stephen C’s review is a good briefing of the history of the place. It’s so cool that it used to be a speakeasy.
You can get in just once a year when the Bexar County Speleological Society opens the place up for a tour. You can climb down dusty passage ways, wriggle through tight spaces to explore new reaches of the cave. It’s a really good time, and I highly recommend checking it out.
The cave is geologically significant because it has several extremely rare species of blind fauna living in it. There’s also supposed to be some sort of rare fern in there too, but I’ve never seen it.
The preservation society has cleaned up a bunch of the spraypaint that ended up there in the ’80’s by teenagers freaking out that they’d never be able to get out of there. For good reason too– back when this place was a speakeasy, two revenuers came and busted the place, and ended up getting lost for 72 hours in the cave’s darkness.

It is a cave, of course it gets 5 stars.
The cave is closed to the public because of endangered critters that live there (Biological studies have discovered several blind invertebrate species found nowhere else on Earth) and to keep it clean due to it being a part of recharging the aquifer. Still a nice little well landscaped park with picnic tables and you can take stairs down in to the sinkhole.
It is the longest known cave in the county with 5340 feet of mapped passages.
Between 1923 and 1933 the cave was a tourist attraction that 300,000 people visited.
During Prohibition a Speakeasy operated in the cave.