Slovak Opal Mines

Slovak Opal Mines are first mentioned in the 15th century and their greatest fame came in the second half of the 19th century when ran by a jeweller from Vienna. To this day the opal mines remain a technical monument.
You can take a guided tour through the mines which takes about an hour. Beware of the signs on the road as it's quite difficult to find this place.
If you are a skilled scuba diver with a certificate there's also a possibility to dive and swim and search for opals in the mines.