Dubník opálové bane – Viliam-akna (TILOS / no diving)
Diving here was stopped: the Slovak mining authority banned entry into the mine adits, and diving was halted on the authority's initiative. The Dubník opal mines between Červenica and Zlatá Baňa near Prešov are among the oldest opal mines in the world, with more than 30 hand-dug adits over 22 km; mining ended in 1922. The flooded part of the Viliam mine had levels at 15, 30 and 50 m linked by shafts, down to 67 m, in acidic (pH 2.8–3.0) water at about 4–5 °C with heavy sediment; after an abnormal water-level drop in 2004 there were collapses underwater. Today the mines are visited on guided dry tours only.
depth: 0–67 m