Alex Mustard / Diver swimming through dense school of Silversides (Atherinidae) inside a coral cavern, Grand Cayman, Cayman Islands, British West Indies, Caribbean Sea. Model released

Jack Dykinga / Roots of Fig trees (Ficus sp) growing in the collapsed Cenote Oxpeejool (sink hole), near Tekax, Yucatan, Mexico. Afternoon light illuminates the elongated cavern on the right. October 2009

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

Jack Dykinga / Cenote Samula (sink hole) with elongated roots of a Fig tree (Ficus sp) from ground level descending to the subterranian tourquoise coloured water below. Rays of sulight pass through the hole to illum...

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

Jack Dykinga / Calcehtok cavern, with its huge circular entrance and Ficus roots descending into the cave, Yucatan, Mexico

Jack Dykinga / Calcehtok cavern, with its huge circular entrance and Ficus roots descending into the cave, Yucatan, Mexico

Jack Dykinga / Calcehtok cavern, with its huge circular entrance with ficus garden and tropical vegetation descending into cave, Yucatan, Mexico

Alex Mustard / Diver exploring a coral cavern, East End, Grand Cayman, Cayman Islands, British West Indies, Caribbean Sea. Model released

Alex Mustard / Diver swimming through dense school of Silversides (Atherinidae) inside a coral cavern, Grand Cayman, Cayman Islands, British West Indies, Caribbean Sea. Model released

Floris van Breugel / Reflected light illuminates this split level view of Zion National Park's Subway formation along the Left Fork. Zion National Park, Utah, USA. November 2009

Tim Laman / Cavern / sinkhole connected to the sea. GEO Expedition ship Shakti visible anchored outside the entrance to the cavern. Sebakor Bay, Fak Fak Peninsula, West Papua, Indonesia, April 2007

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

Steven David Miller / Aboriginal rock art painting of a massive rainbow serpent painted on the roof of a cavern extends deep into the sandstone overhang. Note the people at the bottom of the frame, Main Art Site, Mt Borrad...

Juergen Freund / Massive cave cavern at the end of Gota River in the Caramoan Peninsula, Camarines Sur, Luzon, Philippines 2008.

Juergen Freund / Massive cave cavern at the end of Gota River in the Caramoan Peninsula, Fisherman, Camarines Sur, Luzon, Philippines 2008.

Juergen Freund / Massive cave cavern at the end of Gota River in the Caramoan Peninsula, Camarines Sur, Luzon, Philippines 2008.

Juergen Freund / Massive cave cavern at the end of Gota River in the Caramoan Peninsula, Camarines Sur, Luzon, Philippines 2008.

Thomas Marent / Nest of spiderings, Ankarana Cavern, Ankarana Special Reserve, northern Madagascar

Thomas Marent / Nest of spiderings, Ankarana Cavern, Ankarana Special Reserve, northern Madagascar

Thomas Marent / Bats roosting inside Ankarana Cavern, Ankarana Special Reserve, Madagascar

Jeff Rotman / Coral Reef cavern with sunlight penetrating water. Egypt, Red Sea

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 / Cavern formation, 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

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

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

Colin Monteath / Cavernous weathering in sandstone rock, Sydney, Australia

Roberto Rinaldi / Whitetip soldier fishes (Myripritis vittata) in cavern, Maldives.

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