Britain's Natural History Museum holds a wildlife photography competition every year, with a range of a categories, from plants and fungi to mammals to underwater sights.

It's the last of these categories that make up a new book from the museum, "Unforgettable Underwater Photography." The 160-page book is a collection of underwater photos from the last 50 years of the competition, complete with notes from the photographers about how they captured each stunning image.

Here's a beautiful sampling of pictures from the book, along with how each one came to be as described by the photographer.

'Arctic showtime' by Audun Rikardsen

A male humpback whale swims in the Barents Sea.
(Photo: Audun Rikardsen/Unforgettable Underwater Photography/NHM)

"An exuberant male humpback whale cavorts in the Barents Sea off northern Norway. It's a unique split-level image, shot in low light, freezing conditions, near the end of the polar winter. The humpback is displaying his prowess to both females and rival males, singing as he pirouettes, breaches and dives. It's behavior that increases ahead of the migration south of at least some of the humpbacks who will head to warmer, southern locations, where pregnant females give birth and mating occurs."

'Opening to the underworld' by Christian Vizl

The view from the seafloor of the Aktun Ha cenote in Mexico
(Photo: Christian Vizl/Unforgettable Underwater Photography/NHM)

"The view from below in the still water of the Aktun Ha cenote in Mexico is like looking into a magical aquarium. A natural summer algal bloom colors the top layer of water green, and though the undersides of the ageing lily leaves have been lit by a low light from strobes, they are truly pink. Their stems reach up several meters from the bottom of the wide cenote pool, but though most of them never reach the surface, for much of the year the water is so crystal clear that there is enough light for them to photosynthesize."

'Pearls of spring' by Solvin Zankl

European common toads spawn in a pond in Germany
(Photo: Solvin Zankl/Unforgettable Underwater Photography/NHM)

"It's March — peak migration time for European common toads when most are returning to the ponds and lakes where they were spawned. The move would have begun in autumn, before hibernation, in readiness for the spring push. The temperature now is a trigger, signaling that the water is likely to be ice-free. Here, in a pond in Germany, spawning is in full swing. The photographer is watching the activity on his laptop via a USB wire link to his camera at the bottom of the pond."

'The insiders' by Qing Lin

Three clown fish peek out from a sea anemone
(Photo: Qing Lin/Unforgettable Underwater Photograph/NHM)

"Peeping out of the tentacles of a magnificent sea anemone, off North Sulawesi, Indonesia, are clown anemone fish. They live within the confines of the anemone, immune to its stings, thanks in part to a thick covering of mucus. The social arrangement is such that, not only does their host protect them, but they feed on debris among the tentacles and may even snack on a tentacle. In return, they eat parasites, may drive off harmful fish and aerate the water with their fins; it could even be that they attract fish which the anemone stings and eats."

'The startling warning' by Darryl Torckler

A vibrant red sea urchin off the reefs nearnorthern New Zealand and southern Australia
(Photo: Darryl Torckler/Unforgettable Underwater Photography/NHM)

"Illuminated, this is the world's most spectacular sea urchin, yet it remained undiscovered until 1965. That’s partly because the fiery sea urchin only inhabits deeper reefs off northern New Zealand and southern Australia. But it is also because it hides away in the day, and in deeper water, where red wavelengths are filtered out, it appears black or deep brown rather than fiery red. Its obvious defense, of course, is its needle-like spines."

'Big fish fight' by Jordi Chias Pujol

Two dusky groupers engage in a violent struggle
(Photo: Jordi Chias Pujol/Unforgettable Underwater Photography/NHM)

"These two huge dusky groupers are in a violent struggle. The prize, overlying an undersea volcanic pinnacle off the Azores, is a strip of prime spawning territory into which the owner can attract females. It encompasses a rocky retreat, but the key aspect is that a strong current runs overhead — ideal for carrying away fertilized eggs. Males start out as females, but when they get to be 10-15 years old, and if there aren't too many males around, they change sex."

'Hanging around' by Michel Loup

Male pikes in the pond weeds of a lake in France's Jura Mountains
(Photo: Michel Loup/Unforgettable Underwater Photography/NHM)

"Motionless, as if suspended, young male pike hang close to the surface of a lake-edge creek high in the Jura Mountains of France. Though the water is crystal-clear — filtered from the peaks as it percolates through the limestone — the markings and color of these predators help camouflage them among the pondweed. As solitary hunters, they would normally be spread out in territories, often in places with more plants."

'The beauty of gender-bending' by Alexander Mustard

Two shy hamlets engage in the mating process
(Photo: Alexander Mustard/Unforgettable Underwater Photography/NHM)

"This is a moment of ecstasy, seconds before spawning, in the extraordinary mating ritual of shy hamlets. The male is calling out as he caresses the female, who is curved around him, head-down, about to extrude her eggs. He is fanning his pectoral fins to create a current, which will draw the eggs towards him as he releases his sperm. This coral-reef fish, possibly the most beautiful of the 13 Caribbean species of hamlets, spawns at dusk — a dangerous time, when the most predators are about."

'Big blue mouthful' by Doc White

A blue whale with its throat pouch expanded
(Photo: Doc White/Unforgettable Underwater Photography/NHM)

"This picture was the first ever to show, full frame, a blue whale with its throat pouch expanded, the pleats forced open by the engulfment of a gargantuan amount of water and shrimp-like krill. Having lunged through the krill swarm, the whale is expelling the water, forcing it through the massive sheets of hair-like baleen material, which hang from its mouth. To find large enough aggregations of krill, a blue whale is forced to travel great distances. But when a large swarm is located, the lunge-feeding technique is highly energy-efficient."

"Wildlife Photographer of the Year: Unforgettable Underwater Photography" is published by the Natural History Museum. It is currently on sale in the U.K. (hardback, £20) and will be on sale in the U.S. Sept. 1.