David Fleetham / Humpback whale (Megaptera novaeangliae), mother and calf swimming just below surface in front of the West Maui Mountains. South of Lahaina, West Maui, Hawaii.

David Fleetham / Blacktip reef shark (Carcharhinus melanopterus) swimming just below surface at sunset, split level view. Off the island of Yap, Micronesia.

David Fleetham / Green Sea Turtle (Chelonia mydas) below surf instructor Tara Angioletti on a stand-up paddle board off Canoe Bearch, Maui, Hawaii. Model released.

Juergen Freund / Banded sea kraits (Laticauda colubrina) around the roots of mangrove tree, split level. Mali Island, Macuata Province, Fiji, South Pacific.

Michael Pitts / Lake Assal shoreline - Africas lowest point at 515 feet below sea level, Djibouti, , March 2008

Bryan and Cherry Alexander / Tabular iceberg showing the portion underwater that is sculpted by the sea. Polar regions. Digitally created image composite

Bryan and Cherry Alexander / Ice arch iceberg showing the portion underwater that is sculpted by the sea. Polar regions. Digitally created image composite

Doug Allan / Humpback whale (Megaptera novaeangliae) swimming below a whale watching boat, Sue Flood, photographer in boat, Vava'u, Kingdom of Tonga, South Pacific, September 2006, Model released

Chadden Hunter / Dallol hot springs where water heated by magma laden with potassium, sulphur and iron spews up through salt pans staining the salt deposits with vivid colours, in the bottom of the Rift Valley, 120 me...

Marguerite Smits Van Oyen / Lac Assal, crystallised salt on shoreline, 150m below sea level. Djibouti, East Africa sea water

Alex Mustard / Split level image of hard coral garden flourishing in shallow water below Misool Eco Resort. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean.

David Fleetham / Woman on stand-up paddle board with two curious Humpback whales (Megaptera novaeangliae) in water below. Maui, Hawaii. October 2010. Model released. Digital composite.

Ashley Cooper / Sign at 100 feet below sea level in Death Valley. Death Valley is the lowest, hottest, driest place in the USA, with an average annual rainfall of around 2 inches, some years it does not receive any r...

Ashley Cooper / Canal going from the sea to the reclaimed polder land in Holland, north of Amsterdam, which is some 20 feet lower than the sea. With 50% of the Netherlands below sea level, it is incredibly vulnerable...

Ashley Cooper / Houses in Almere with solar PV panels on the roof. It is the Netherlands youngest town, in Flevoland, which was reclaimed from the sea. This planned city is very green, with space heating provided fro...

Ashley Cooper / Sun Island in Almere, which is the Netherlands youngest town, in Flevoland, which was reclaimed from the sea. This planned city is very green, with space heating provided from a nearby combined heat a...

Ashley Cooper / Sun Island in Almere, which is the Netherlands youngest town, in Flevoland, which was reclaimed from the sea. This planned city is very green, with space heating provided from a nearby combined heat a...

David Fleetham / Green Sea Turtle (Chelonia mydas) below surf instructor Tara Angioletti on a stand-up paddle board off Canoe Bearch, Maui, Hawaii. Model released.

Alex Mustard / A Caribbean reef shark (Carcharhinus perezi) just below the surface. Split level with blue sky and clouds. Long Island, Bahamas. Bahamas Sea, Tropical West Atlantic Ocean.

David Tipling / Split level view of above and below the water on the Polperro coast Cornwall, UK July

Franco Banfi / Roca Partida above and below sea level with divers - a small rock island covered in white bird guano in the Revillagigedo Archipelago Biosphere Reserve, Socorro Islands, Western Mexico. Composite s...

Pete Oxford / Man walking along sea wall, built by the Dutch as town is below sea level, Georgetown, Guyana, South America

Juergen Freund / Split level of shallow coral reef, Rowley Shoals, Western Australia

Chris & Monique Fallows / Shy Albatros (Thalassarche cauta), Cape Point, South Africa, December.

Alex Mustard / A pair of blue sharks (Prionace glauca) are revealed below the surface of the sea, as a camera is dipped through the surface. Penzance, Cornwall, England, British Isles. English Channel, North East At...

Michael Pitts / Lake Assal - Africas lowest point at 515 feet below sea level , with dense concentrations of salt on the shoreline, Republic of Djibouti. March 2008

Michael Pitts / Lake Assal shoreline - Africas lowest point at 515 feet below sea level, Djibouti, , March 2008

Michael Pitts / Lake Assal shoreline - Africas lowest point at 515 feet below sea level, Djibouti, March 2008

Michael Pitts / Lake Assal shoreline - Africas lowest point at 515 feet below sea level, Djibouti, March 2008

Michael Pitts / Lake Assal shoreline - Africas lowest point at 515 feet below sea level, Djibouti, March 2008

Michael Pitts / Lake Assal - Africas lowest point at 515 feet below sea level, Djibouti, March 2008

Michael Pitts / Lake Assal - Africas lowest point at 515 feet below sea level, with dense concentrations of salt on the shore line, Djibouti, March 2008

Nick Upton / Split level view of Spiny starfish (Marthasterias glacialis) among kelp just below the waterline on a low spring tide, Wembury, Devon, UK, August.

Jack Dykinga / Crystalized salt formation across the Death Valley National Park valley floor with morning light on Badwater, the lowest point in the US at 86m below sea level with Telescope Peak in the Panamint Moun...

Alex Mustard / Cushion sea star (Oleaster reticulatus) on turtle grass (Thalassia testudinum) below man standing on a jetty, East End, Grand Cayman, Cayman Islands, British West Indies. Caribbean Sea.

Alex Hyde / Red algae living just below the surface of snow, known as 'Red Snow'. The red pigment provides protection from ultra-violet light. Austrian Alps, 2800 metres above sea level, June 2009.

Bryan and Cherry Alexander / Melting iceberg showing the portion underwater that is sculpted by the sea. Polar regions. Digitally created image composite

Bryan and Cherry Alexander / Melting iceberg showing the portion underwater that is sculpted by the sea. Polar regions. Digitally created image composite

Pete Oxford / Aerial view of the sea wall protecting Georgetown, a city built below sea level, Guyana, December 2009

Pete Oxford / Aerial view of Georgetown, capital of Guyana, a city built on the coast below sea level, Guyana, December 2009

Floris van Breugel / Rare desert rains flood the salt flats of Badwater basin, Death Valley National Park. Badwater is the lowest point in North America at 282 feet below sea level. California, USA, January 2010.

Chadden Hunter / Dallol hot springs where water heated by magma laden with potassium, sulphur and iron spews up through salt pans staining the salt deposits with vivid colours, in the bottom of the Rift Valley, 120 me...

Chadden Hunter / Dallol hot springs where water heated by magma laden with potassium, sulphur and iron spews up through salt pans staining the salt deposits with vivid colours, in the bottom of the Rift Valley, 120 me...

Chadden Hunter / Cameraman, Martyn Colbeck, filming the Dallol hot springs where water heated by magma laden with potassium, sulphur and iron spews up through salt pans staining the salt deposits with vivid colours, i...

Harley Betts / Active volcanic vent 90 meters below sea level, White Island, Bay of Plenty, New Zealand

George Chan / Mud patterns on the vast expanse of Aydingkol Lake, Xinjiang Province, North-west China. This is the second lowest point on earth at 154m below sea-level. August 2006, BBC "Wild China" series

Dan Burton / Diver underwater just below surface, split level with person standing on boat deck, Egypt

Marguerite Smits Van Oyen / Afar tribesman mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level.

Marguerite Smits Van Oyen / Afar tribesmen mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level. Sea water seeps through into lake and evaporates.

Marguerite Smits Van Oyen / Afar tribesmen mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level. Sea water seeps through into lake and evaporates.

Marguerite Smits Van Oyen / Crevasse where The Rift enters Africa at Lac Assal (150m below sea level) Djibouti, Africa New

Marguerite Smits Van Oyen / Lac Assal, crystallised salt on shoreline, 150m below sea level. Djibouti, East Africa. sea water

Marguerite Smits Van Oyen / Lac Assal, crystallised salt on shoreline, 150m below sea level. Djibouti, East Africa sea water

Marguerite Smits Van Oyen / Hot spring sea water seeping into Lac Assal 150m below sea level. Djibouti, East Africa

Marguerite Smits Van Oyen / Afar tribesmen mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level. sea water

Marguerite Smits Van Oyen / Afar tribesman loading salt onto camel. Lac Assal, Djibouti, East Africa. 150m below sea

Marguerite Smits Van Oyen / Lac Assal, crystallised salt on shoreline, 150m below sea level. Djibouti, East Africa sea water

Marguerite Smits Van Oyen / Hot spring sea water seeping into Lac Assal 150m below sea level. Djibouti, East Africa

Marguerite Smits Van Oyen / Lac Assal, crystallised salt on shoreline, 150m below sea level. Djibouti, East Africa sea water

Marguerite Smits Van Oyen / Afar tribesmen mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level. sea water

Marguerite Smits Van Oyen / Afar tribesman mining salt, Lac Assal, Djibouti, East Africa. 150m below sea level. sea water

Harley Betts / Active volcanic vent 90 meters below sea level, White Island, Bay of Plenty, New Zealand

Henley Spiers / Northern gannet (Morus bassanus) with fish in beak whilst sitting on water's surface as other birds dive below, Shetland, Scotland, UK, North Sea. Split level shot.

Alex Mustard / Coral reef below the cliffs of Shark Observatory, Ras Mohammed National Park, Sinai, Egypt, Red Sea.

David Hall / Bull kelp (Nereocystis luetkeana) above and below sea surface, Queen Charlotte Strait, British Columbia, Canada. September

David Hall / Split level shot of kelp and other 'seaweeds' partly exposed at low tide, with coniferous forest in the background. Seen in this photograph are the following: Turkish washcloth (Mastocarpus pa...

Willem Kolvoort / Split level view of Epaulette surgeon fish (Acanthurus nigicauda) below coral 'mushroom' half above sea surface, with Frigate birds flying above, Johnny Channel, Aldabra, Indian Ocean