Andy Murch / Nurse shark (Ginglymostoma cirratum) swimming below surface at sunset. South Bimini Island, Bahamas.

Andy Murch / Lemon shark (Negaprion brevirostris) surfacing with open mouth. Tiger Beach, Little Bahama Bank, Bahamas.

Ole Jorgen Liodden / Polar bear (Ursus maritimus) walking on drifting sea ice. Svalbard, Norway.

Brandon Cole / Amazon river dolphin (Inia geoffrensis) split level view showing playful dolphin with mouth open. Rio Negro, Brazil.

Nick Hawkins / Atlantic sturgeon (Acipenser sturio), Minas Basin of the Bay of Fundy, Canada. July 2019.

David Hall / Split level of a Sockeye salmon (Oncorhynchus nerka) in shallow water; Adams River, British Columbia, Canada. October.

David Hall / Split level of Sockeye salmon (Oncorhynchus nerka) during migration; Adams River, British Columbia, Canada. October.

Alex Mustard / Hard corals (including Acropora sp., Platygyra sp. and Pocillopora spp.) growing in shallow water and reflected in the surface. Ras Umm Sid, Sharm El Sheikh, Sinai, Egypt. Red Sea

Alex Mustard / Southern stingray (Dasyatis americana) two swimming over sand bar, under stormy sky. Grand Cayman, Cayman Islands. British West Indies.

Alex Mustard / Split level image of Leather corals (Sarcophyton sp.) flourishing in shallow water close to uninhabited coral islands. Misool, Raja Ampat, West Papua, Indonesia. Ceram Sea. Tropical West Pacific Ocean...

Jordi Chias / Leopard seal (Hydrurga leptonyx) resting over an iceberg, Antarctic Peninsula, Antarctica.

Jordi Chias / Iceberg, Antarctic Peninsula, Antarctica.

Jordi Chias / Weddell seal (Leptonychotes weddellii), Antarctic Peninsula, Antarctica.

Nick Hawkins / Chum salmon (Oncorhynchus keta) migrate up a small river near Bella Bella, British Columbia, Canada. September

SCOTLAND: The Big Picture / Northern gannets (Morus bassanus) diving for fish, split level shot, Shetland, Scotland, UK, July.

SCOTLAND: The Big Picture / Northern gannets (Morus bassanus) diving for fish, split level shot, Shetland, Scotland, UK, September.

SCOTLAND: The Big Picture / Guillemot (Uria aalge) swimming, split level view, Shiant Isles, Outer Hebrides, Scotland, UK. June, 2018.

Klein & Hubert / Piglet swimming in turquoise sea. Bahamas.

Doug Perrine / Dugong or Sea cow (Dugong dugon) split level view, Calauit Island, off Busuanga, Calamian Islands, Palawan, Philippines.

Shane Gross / Lemon shark pup (Negaprion brevirostris) in mangrove forest which acts as a nursery for juveniles of this species. Eleuthera, Bahamas.

Tui De Roy / Marine iguana (Amblyrhynchus cristatus), Punta Espinosa, Fernadina Island, Galapagos

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 / Whale shark (Rhincodon typus), a commercial encounter for tourists with feeder above on canoe. Oslob, Cebu, Philippines. April 2017.

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

Alex Mustard / Giant mottled eel (Anguilla marmorata) in river with villagers washing clothes in background. Ambon, Maluku Archipelago, Indonesia. Banda Sea, tropical west Pacific Ocean.

Jordi Chias / Woman snorkeller swimming inside lagoon Toau atoll, Tuamotu Archipelago, French Polynesia, Pacific Ocean.

Alex Mustard / American crocodile (Crocodylus acutus) reflected in the surface as it floats, Jardines de la Reina, Gardens of the Queen National Park, Cuba. Caribbean Sea.

Alex Mustard / Southern stingray (Dasyatis americana) swimming over sand in shallow water, split level photo with blue sky and clouds. The Sandbar, Grand Cayman, Cayman Islands. British West Indies. Caribbean Sea.

Alex Mustard / RF - Southern stingray (Dasyatis americana) swimming over sand in shallow water at dawn, Cayman Islands, Caribbean Sea. (This image may be licensed either as rights managed or royalty free.)

Tui De Roy / Split level view of Brown pelicans (Pelecanus occidentalis) feeding, Tortuga Bay, Santa Cruz Island, Galapagos

Theo Bosboom / Split level view of autumn leaves underwater and tree reflections on the water surface, Levenumse beek, the Netherlands, November.

Klein & Hubert / European hedgehog (Erinaceus europaeus) swimming in pond, split level view, France. Controlled conditions.

Alex Mustard / Grey seal (Halichoerus grypus) split level of young seal investigating the camera with its nose and whiskers, close to shore. Lundy Island, Devon, UK, Bristol Channel. August

Claudio Contreras / American crocodile (Crocodylus acutus) split level with animal resting at surface in shallow water over seagrass bed, Banco Chinchorro Biosphere Reserve, Caribbean region, Mexico

Alex Mustard / Grey seal (Halichoerus grypus) split level view of young investigating camera lens, Lundy Island, Devon, UK, Bristol Channel, August

SCOTLAND: The Big Picture / Gannets (Morus bassanus) diving to feed on discarded fish, Shetland, Scotland, UK, April.

SCOTLAND: The Big Picture / Gannets (Morus bassanus) diving to feed on discarded fish, Shetland, Scotland, UK, April.

SCOTLAND: The Big Picture / Gannets (Morus bassanus) diving to feed on discarded fish, Shetland, Scotland, UK, April.

Chris & Monique Fallows / Great white shark (Carcharodon carcharias) split level view, New Zealand, March.

Ingo Arndt / American Alligator (Alligator mississippiensis) split level, Everglades, USA, January.

Ingo Arndt / American alligator (Alligator mississippiensis) split level, Everglades, USA, January.

Tony Wu / Juvenile Australian sea lion (Neophoca cinerea) resting in shallows, possibly to thermoregulate, Carnac Island, Western Australia.

Jordi Chias / Split level view of Blue shark (Prionace glauca) at surface at sunset, Azores Islands, Portugal, Atlantic Ocean

Willem Kolvoort / Split level view of young mangrove shoots, Passe Grande Magnan / Magnan channel, Aldabra, Indian Ocean

Willem Kolvoort / Aldabra giant tortoise (Aldabrachelys gigantea) swimming in Passe Grande Magnan / Magnan channel, Aldabra, Indian Ocean Image taken under controlled conditions.

Willem Kolvoort / Split level view of Aldabra giant tortoise (Aldabrachelys gigantea) swimming in Passe Grande Magnan / Magnan channel, Aldabra, Indian Ocean Image taken under controlled conditions.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) low angle perspective of open bills whilst feeding, Lake Kerkini, Greece, February.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) low angle perspective of open bills whilst feeding, Lake Kerkini, Greece, February.