Kamionka Piast (Opole)
Kamionka Piast is a former marl pit in Opole dating from 1906 (cement works), with turquoise water. A Dutch guide quoted by search gives max depth 15 m and visibility 4-10 m. There are two shore entries: an easy shallow one and one reached by a driveway and a short forest walk. The pit is large, so watch your gas.
depth: 0–15 m