Sơn Đoòng cave in Phong Nha-Kẻ Bàng National Park, Vietnam (© David A Knight/Shutterstock)

Beach huts covered in snow in Brighton and Hove, England (© Tim Jones/Alamy)

Roaring Fork in Great Smoky Mountains National Park, Tennessee, USA (© Bernie Kasper/Getty Images)

Diamond Beach across from Jökulsárlón, a glacier lagoon in Iceland (© surangaw/Getty Images)

Arkady Shilkloper plays his three-metre-long alphorn atop the Sydney Opera House as part of the 2013 Sydney Festival (© Mark Metcalfe/Getty Images)

White Cliffs of Dover, England (© LisaValder/Getty Images)

Foucault pendulum at the Panthéon in Paris, France (© Adolf/Adobe Stock)

Spanish shawl nudibranch on kelp off Santa Catalina Island, Channel Islands, California, USA (© WaterFrame/Alamy)

Huanglong Scenic and Historic Interest Area in Sichuan, China (© primeimages/Getty Images)

Huanglong Scenic and Historic Interest Area in Sichuan, China (© primeimages/Getty Images)

Antique iceboats on the frozen Hudson River near Astor Point in Barrytown, New York, USA (© Mike Segar/REUTERS)

Flamingos in the Eduardo Avaroa Andean Fauna National Reserve in Bolivia (© Art Wolfe/Danita Delimont)

High-angle view of man surfing in the sea, New South Wales (© Benjamin Lee/EyeEm/Getty Images)

Château de Beynac overlooking the Dordogne Valley in France (© Gareth Kirkland/Alamy)

Glass igloos at the Kakslauttanen Arctic Resort in Saariselkä, Finland (© Lingxiao Xie/Getty Images)

Agoyán waterfall near Baños de Agua Santa, Ecuador (© Laura BC/Getty Images)

Agoyán waterfall near Baños de Agua Santa, Ecuador (© Laura BC/Getty Images)

Ruins of the ancient Mayan city of Calakmul surrounded by the jungle, Campeche, Mexico (© Alfredo Matus/Shutterstock)

Gentoo penguins near Danco Island, Antarctica (© David Merron/Getty Images)

Red squirrel in the Scottish Highlands, Scotland (© Scotland: The Big Picture/Minden Pictures)

Railroad crossing Bloody Brook alongside the Connecticut River in Hanover, New Hampshire, USA (© DEEPOL by plainpicture)

Chu Dang Ya near Pleiku in Gia Lai Province, Vietnam (© Quang Ngoc Nguyen/Alamy)

Handicrafts of the Guna people of the Guna Yala archipelago, Panama (© Lars Schneider/Tandem Stills + Motion)

Lightning strikes near rock formation, Church Rock, Red Rock Park, New Mexico, USA (© Tim Fitzharris/Minden Pictures)

Heart Reef, part of the Great Barrier Reef off Queensland, Australia (© Peter Adams/Offset by Shutterstock)

Taormina, Sicily, Italy (© Juergen Schonnop/Getty Images)

Kermits Pool, Karijini National Park, Western Australia (© Abstract Aerial Art/Getty Images)

European robin during a winter snowstorm, Peak District National Park, England (© Ben Robson Hull Photography/Getty Images)

Svalbard Global Seed Vault with a glittering facade designed by artist Dyveke Sanne, Svalbard, Norway (© Pal Hermansen/Minden Pictures)

Mittenwalder Hütte in the Bavarian Alps of Germany (© Sebastian Frölich/Offset by Shutterstock)

Panoramic view of the city of Toledo after sunset, Spain (© Frank Fischbach/Alamy)