Deep Fresh water Wall
Named straight and simple - Goluboe Ozero means Blue Lake.
Small and extremely deep karst cavity : 256 meters maximum depth (40 meters avg) and just 15 minutes to walk around.
The water is blue indeed and absolutely transparent: from 40 m depth you can still see crowns of the trees on the banks.
Dry suite is needed (the water is ever cold 6-9 degrees Celsius).