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

Polar bear in waters off Svalbard, Norway (© Westend61/Getty Images)

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

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

Sunset over Nash Point on the Glamorgan Heritage Coast, South Wales in winter. (© AWL Images/DanitaDelimont.com)

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

Crater Lake in Oregon, USA (© Steve Bloom Images/Alamy)

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

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

Tre Cime di Lavaredo as seen from the Lavaredo fork, Sexten Dolomites, Italy (© AWL Images/Danita Delimont)

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

Aït Benhaddou, Atlas Mountains, Morocco (© Alex Cimbal/Shutterstock)

Frosty morning at Hadrian's Wall on Winshields Crags, looking towards Crag Lough in Northumberland (© Nigel Eve/Shutterstock)

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

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

Château de Beynac overlooking the Dordogne Valley, 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)

Sunset over an abandoned cottage on the foothills of Brown Willy on Bodmin Moor, the highest point in Cornwall (© Helen Hotson/Alamy Stock Photo)

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

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

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

View over the village of Gößweinstein and Gössweinstein castle, Bavaria, Germany (© Juergen Sack/Getty Images)

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

Twilight view over Edinburgh as seen from Calton Hill (© Henk Meijer/Alamy Stock Photo)

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

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

Arctic wolf family in Canada (© 4FR/Getty Images)

Robin during a winter snowstorm, Peak District National Park, Derbyshire (© 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 Hut in the Bavarian Alps of Germany (© Sebastian Frölich/Offset by Shutterstock)