Rummu - Prison Beneath the Surface
Rummu Karjäär is located in Estonia. Rummu used to be a Soviet quarry but after the collapse of the Soviet union, the prison/quarry was abandoned. The pumps stopped and the prison slowly filled with water.
There is something abandoned both above and under the surface. Above the waterline, one can find old concrete buildings, guard towers and barbed wire. The really cool stuff is under the surface. There are many buildings, fences, hangars and walls hidden beneath the waterline.