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

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

Aerial view of Sindhudurg Fort, Maharashtra coast (© MovingMaratha/Alamy)

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

Red-fronted macaws in Omerque, Cochabamba, Bolivia (© Bernard Castelein/Minden Pictures)

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)

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

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

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

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)

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

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

Indian army soldiers march during a dress rehearsal for the Republic Day parade (© Adnan Abidi/Reuters)

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

Weather Station and Fichtelberghaus Hotel on Fichtelberg Mountain (1214m), Oberwiesenthal, Ore Mountains, Erzgebirge, Saxony, Germany (© Cornelia Dorr/eStock Photo)

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)

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

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)

CST station Mumbai, illuminated for Republic Day (© R Modi/Dinodia Photo/age fotostock)

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

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

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

School children pay homage to a portrait of Mahatma Gandhi on Martyr's Day. (© ARUN SANKAR/AFP via Getty Images)

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

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