Slovak opal mines are located in eastern Slovakia near Prešov. There are approximately 22 km of mining tunnels, shafts and adits that were drilled through the hills like Emmentaler cheese, some up to a depth of over 150 meters. Entry to this area of the museum is adapted in a way to preserve it for future generations of visitors. The location is a protected region of the country and a protected deposit area. Slovak opal mines are also an important wintering place for thousands of bats, of which there are many species.