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-...