A woman travels on a train stopping at a subway station visited by foreign reporters in central Pyongyang, North Korea on April 14, 2017. (Photo by Damir Sagolj/Reuters)

Teeming with images of spectacular underwater scenes from around the world, Call of the Blue is the culmination of a five-year project by the photographer and ocean conservationist Philip Hamilton. This groundbreaking book includes contributions from acclaimed scientists and ocean “guardians”, who reveal what drove them to answer the call of the blue. (Photo by Philip Hamilton/The Guardian)

These otherworldly images give a rare glimpse inside caves barely ever seen by the human eye. With its peculiar pools of water and strange colored sediments, the bizarre looking tunnels of the Shakuranskaya cave could be a set straight out of a science fiction film. Found in the disputed region of Abkhazia, around 75 miles outside of Sochi, Russia, it is rumored the incredible underground chambers were formed after a huge earthquake struck the area in 1892. After causing part of a nearby mountain to collapse into the Amtkel River, it is thought the dam-like affect has created a series of interconnecting underground tunnels. Here: a large pool of water inside one of the tunnels. (Photo by Vladimir Mulde/Caters News)

High winds blow sand across the highway near the original north shore of Owens Lake, now miles from the nearest pool of water, on May 5, 2007 near Lone Pine, California. (Photo by David McNew/Getty Images)

“The king of herrings or giant oarfish, an oarfish of the family Regalecidae, is the world's longest bony fish. Its total length has been documented to reach 17 m (56 ft), and it can weigh up to 300 kg (661 lb)”. – Wikipedia

Wildlife photographer Danté Fenolio has headed into areas untouched by sunlight – deep seas, caves and underground – and found creatures that are exploding with colour. Here: The golden harlequin toad has vanished from the wild, and only a small number live on in captivity. A fungus caused them, and many other amphibians, to die out in their home in Central America. (Photo by Danté Fenolio/The Guardian/Johns Hopkins University Press)

Machu Picchu, Peru. A llama’s-eye view of the legendary Inca settlement at Machu Picchu, isolated high in the Peruvian Andes. (Photo by Jim Turner/National Geographic)

The sea leopard. The Antarctic Peninsula. (Photo by Paul Nicklen)