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

Juan Carlos Munoz / Stalagtites in El Soplao cave, Cantabria, Spain July.

Alex Hyde / Assorted stalagmites and stalagtites and other speleothems (mineral deposits formed from groundwater) at Postojna Cave, Slovenia. April 2016.

Alex Hyde / Assorted stalagmites and stalagtites and other speleothems (mineral deposits formed from groundwater) at Postojna Cave, Slovenia. April 2016.

Alex Hyde / Assorted stalagmites and stalagtites and other speleothems (mineral deposits formed from groundwater) at Postojna Cave, Slovenia. April 2016.

Robin Chittenden / Poole's Cavern. Buxton Peak District, Derbyshire, UK, December.

Chadden Hunter / Cameraman Gavin Thurston filming ice crystals under volcanic ice cave, Mount Erebus, Antarctica. Taken on location for BBC Frozen Planet series, 2009

Chadden Hunter / Filming around the extremely delicate ice formations in volcanic ice cave, Mount Erebus, Antarctica. Taken on location for BBC Frozen Planet series, 2009

Pete Oxford / Cenote, a freshwater-filled limestone sinkhole, with stalagtites and stalagmites. Quintana Roo, Yucatan Peninsula, Mexico.

Robert Smith / Freshwater Limestone sinkhole / Cenote called 'Sistema Mayan Blue' with Stalagtite formations, Tulum, Quintana Roo, Mexico, September 2008

Robert Smith / Diver exploring a freshwater Limestone sinkhole / Cenote a traverse to 'The White River' with stalactites and stalagmites, Tulum, Quintana Roo, Mexico, September 2008

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