Jewel Cave National Monument, USA

The Jewel Cave National Monument in South Dakota was discovered by brothers Frank and Albert Michaud in 1900. Passing through the Black Hills on their way back from Alaska, they heard an unusual noise. They discovered that it was due to cold air coming out of a small hole. Blasting the entrance with dynamite, they were amazed by the wonderful underground galleries covered with calcite formations that glittered like gems. In 1908, President Roosevelt declared the cave a national monument. Only in 1939, it became accessible to tourists. At first only half a mile of the cave's length was explored, now exploration has reached greater lengths. The Jewel Cave National Monument is now considered the third longest cave in the world. Its walls are decorated with glittering calcite crystals, stalactites and stalagmites.