There's a sandstone quarry I've swam in, the deep end is 60+ feet deep. I wouldn' want to be a lifeguard there. Although the added depth doesn't really mater past about 15 feet.

https://www.cityoftenino.us/parks-recreation/page/quarry-poo...

This quarry in chicago used as a storm water buffer is cool. It would be interesting to drive over it when it is full.

https://chicago.suntimes.com/environment/2026/07/11/chicago-...

There are lots of these in northern Minnesota and they used the tailings to turn the areas into some really great mountain biking trails, which really helps those old mining towns.