Coral substrate forming an underwater cave, Ras Mohammed National Park, Sharm El Sheikh, Red Sea, Egypt, North Africa, Africa

A myriad of hard and soft corals compete for space on the substrate of the snorkel site known as the Milky Way, Palau, Micronesia, Pacific

A myriad of hard and soft corals compete for space on the substrate of the snorkel site known as the Milky Way, Palau, Micronesia, Pacific

A myriad of hard and soft corals compete for space on the substrate of Darwin's Wall, Palau, Micronesia, Pacific

A myriad of hard and soft corals compete for space on the substrate of the snorkel site known as the Milky Way, Palau, Micronesia, Pacific

Green sea turtle (Chelonia mydas), feeding on the sand near Caye Caulker, inside the Mesoamerican Barrier Reef, Belize

Split image of sandy substrate and beach, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Split image of snorkeller hovering over sandy substrate, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Snorkeller dives down to observe the substrate, St. Peter and St. Paul's rocks, Brazil, South America

Coral bommies in sandy substrate, Rongelap, Marshall Islands, Micronesia, Pacific

Snorkeller dives down to observe the substrate, St. Peter and St. Paul's rocks, Brazil, South America

Split image of snorkeller hovering over sandy substrate, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Split image of sandy substrate and beach, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Split image of rocks, lighthouse and underwater substrate, St. Peter and St. Paul's rocks, Brazil, South America

Split image of sandy substrate and beach, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Split image of snorkeller hovering over sandy substrate, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Small Tube Worm (Pomatocerous triqueter), group of tiny colourful individuals on rocky substrate, St Abbs, Scotland, UK North Sea

Split image of sandy substrate and beach, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Underwater substrate, St. Peter and St. Paul's rocks, Brazil, South America

Split image of sandy substrate and beach, Hanauma Bay, Oahu, Hawaii, United States of America, Pacific

Underwater substrate, St. Peter and St. Paul's rocks, Brazil, South America

Underwater substrate, St. Peter and St. Paul's rocks, Brazil, South America

Split image of rocks and underwater substrate, St. Peter and St. Paul's rocks, Brazil, South America

Slender seahorse (Hippocampus reidi) sitting on substrate, Ponta de Sao Vicente, Isabella Island, Albemarle, Galapagos Islands, a UNESCO World Natural Heritage Site, Ecuador, Pacific Ocean

Rocky substrate, in the first few meters below the surface of the ocean. Volcanic underwater bottoms. La Gomera, Canary Islands.