There are some very nice natural swimming pools in the local area thanks to that, for example:
https://www.kudyznudy.cz/aktivity/koupaliste-u-libuse-v-lule... (stone quarry, IMHO one of the best summer swimming pools anywhere)
https://cs.wikipedia.org/wiki/Zatopen%C3%BD_lom_v_Blansku (stone quarry)
https://www.jednoustopouceskem.cz/listing/lom-kaolinka-prime... (kaolin mine)
Red Bull and Tesla use a lot of water in the region, further stressing the situation.
it turns out that all of the things leaching into the water made it quite acidic and relatively toxic!
so, y'know, probably good to check what kind of mine it was before going for a swim!
Over here, the seas that are public for swimming are usually checked. Not just for acidic things from a mine. But in summer, when it's hot, for cyanobacters. And then they warn (e.g. on radio, in local TV news like "Hessenschau", on the web. And if there are organized beaches, then even with posters.
We have few actually dangerous ones. After a search, I only found the "Silbersee" in Bayern. That one has sulfuric fumes. And that reminded me that some vulcanic "Maar" sees in the Eifel (area next to Belgium) might be dangerous to to sulfur and CO2 since they are still a bit vulcanic active.
Ex-seas from mining companies that are still toxic are fenced off and not accessible.
Maybe if you're a US reader you don't know about the "fencing culture" difference between USA and Europe. In the USA, basically everything is behind a fence. I drove e.g. in Texas, New Mexico, Maryland: even the corn fields were behind barbed wire or other fences. This is totally not the case in Germany. Here by default things are accessible. Fields, Meadows, Forests ... by default no fences. Even if e.g. the forrest is private property. So if you, as a german, come to an area where the lake is actually fenced around, then this speaks loudly to you and your complications sensor.
An interesting 'Germanism': German 'see' translates to English 'lake', German 'meer' translates to English 'sea'. It is even more interesting when comparing German to Dutch:
German 'see' -> Dutch 'meer'
German 'meer' -> Dutch 'zee' (pronounced almost like German 'see')
[1] https://www.youtube.com/watch?v=LAL_d-5FsGQ - Tioga Pit Mine (225' deep)
[2] https://en.wikipedia.org/wiki/Portsmouth_Mine_Pit_Lake (450' deep)
That said, the Lake of Constance (shared by Switzerland, Germany and a bit of Austria) is at the deepest location 251 m deep. There is however nothing terrifying about that. And it's about twice as deep as your Portsmouth Mine.
Outside just the marvelous part industrial, part natural beauty, there are so many interesting little details:
- that original ~90 years old water slide made from a mixture of cement and marble that has been perfectly smoothed out (locals can do insane tricks on it)
- a built-in olympic sized swimming pool integrated into the back of the quarry by constructing 2 parallel edges with jump points and eyes for attaching swim lane markers
- vast meadows around the quarry on which you can put your towel and sunbathe or just read a book when not swimming
- the place itself on the edge of a forested higland that changes into a flat plane with fields and wine gardens - they are quite a few viewing spots where you can see far and wide into the landscape
And indeed, the Panelka bistro right next to the quarry is superb and has won quite a few prices for its food over the years - and they host many interesting cultural events as well! :)