Robert Smith / Diver exploring a freshwater Limestone sinkhole / Cenote called 'The Pit' - a traverse to 'The Blue Abyss' with stalactites and stalagmites, Tulum, Quintana Roo, Mexico, September 2008. Model released

Robert Thompson / Long John Silver stalactite in the Marble Arch Caves, Enniskillen, County Fermanagh, Northern Ireland, UK, January 1995

Jack Dykinga / Stalagmites and stalactites in underground cavern with tourquise waters of Cenote Papakal (sink hole), near Merida, Yucatan, Mexico October 2009

Claus Meyer / Visitors explore an expansive cavern with stalactites, Vale Do Peruacu, Minas Gerais, south Cerrado savannah, Brazil

John Cancalosi / Rhodochrosite (manganese carbonate) in a polished slice of stalactite. Argentina

Chien Lee / Sunlight filters through lowland rainforest into the entrance of a limestone cave, Gunung Mulu National Park, Malaysia

Robert Thompson / Stalactite formations in the Marble Arch Caves, County Fermanagh, Northern Ireland, UK, January 1995

Steven David Miller / Ngilgi Cave is in a system of limestone caves richly decorated with the calcium carbonate formations including this stalactite tipped with a cluster of crystals. The clear straw at the base is man-ma...

Jack Dykinga / Stalagmites and stalactites in underground cavern with azure waters of Cenote Papakal (sink hole), near Merida, Yucatan, Mexico October 2009

John Cancalosi / Rhodochrosite (Manganese carbonate mineral) polished slice of stalactite, Argentina

Ingo Arndt / Stalactites, Carlsbad Caverns National Park, New Mexico

Ingo Arndt / Stalactites, Carlsbad Caverns National Park, New Mexico

Ingo Arndt / Stalactites, Carlsbad Caverns National Park, New Mexico

Albert Lleal / Salt crystal stalactites in the salt mines of Cardona, Barcelona, Spain

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Martin Gabriel / King Otto cave, Velburg, Bavaria, Germany, dripstones with mark of ice-age water level

Martin Gabriel / King Otto cave, Velburg, Bavaria, Germany, stalactite and stalagmite grown together

Norbert Wu / Scuba diver exploring sea ice stalactites or brine channels, Antarctica

Gerry Ellis / Interior of Kelly Hill caves, Kangaroo Island, South Australia

Jim Brandenburg / Stalagmites and stalactites in Lehman Caves, Great Basin National Park, Nevada

Jim Brandenburg / Stalagmites and stalactites in Lehman Caves, Great Basin National Park, Nevada

Dietmar Nill / Lesser horseshoe bat roosting on stalactite in cave, Germany {Rhinolophus hipposideros}

Roberto Rinaldi / Diver in Chandelier cave, Palau.

Rick Tomlinson / The crew of Skip Novak's yacht "Pelagic" explore an ice cavern in Antarctica.

John Cancalosi / Rhodochrosite (MnCO3), an ore of manganese. Cross-section of a stalactite. Argentina

Chien Lee / Limestone formations within Lang Cave, Gunung Mulu National Park, Malaysia

Chien Lee / Limestone formations within Lang Cave, Gunung Mulu National Park, Malaysia

Robert Thompson / Martel stalactite in the Marble Arch Caves, County Fermanagh, Northern Ireland, UK, January 1995

Robert Thompson / Long John Silver stalactite in the Marble Arch Caves, Enniskillen, County Fermanagh, Northern Ireland, UK, January 1995

Ingo Arndt / Calcite flowstone formations, Carlsbad Caverns National Park, New Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Ingo Arndt / Stalactites in cave, Cacahuamilpa Caverns, Grutas de Cacahuamilpa National Park, Guerrero, Mexico

Gerry Ellis / Harrison's Cave showing stalactites and stalagmites, Barbados, Caribbean

Jim Brandenburg / Stalactites at Lehman Caves, Great Basin National Park, Nevada

Norbert Wu / Norbert Wu photographing under the ice in Antarctica on shelf of an iceberg, brine channel stalactite and sea ice ceiling