A visitor takes pictures with a smartphone of an artwork entitled “Blue Europe, 2015” by Chinese artist Liu Bolin, on display during the exhibition “Visible Invisible” at the Mudec Museum in Milan on May 14, 2019. Liu Bolin known as “the invisible man” for his photographic self-portraits, focused on a body-painting blending with the surrounding area. (Photo by Miguel Medina/AFP Photo)

A leather worker with his frightening types of clothes prepares after painted themselves using chimney soot as they participate in a programme within the September 6 celebrations, in Balikesir, Turkey on September 06, 2019. According to the story, “Tulutabaklar” were painting themselves with ash and wearing goatskin or sheep skin to frighten the Greek soldiers occupying Balikesir during the National Struggle period. Every year on September 6, Tulutabaklar perform this ritual as part of the celebration of liberation of the city for 96 years. (Photo by Ali Atmaca/Anadolu Agency via Getty Images)

Children play amid icebergs on the beach in Nuuk, Greenland, June 5, 2016. (Photo by Alister Doyle/Reuters)

Tube passengers take a selfie next to a London underground roundel designed for the launching of the Night Tubes on display at Oxford Circus underground station in London, Britain, 19 August 2016. The long delayed Night Tubes service launched on 19 August. The 24-hour service will operate on Fridays and Saturdays. (Photo by Hannah Mckay/EPA)

People ride a boat on the Tianquan Lake during a foggy day in Xuyi in China's eastern Jiangsu province on November 20, 2019. (Photo by AFP Photo/China Stringer Network)

Andrey Karr from Western Riders Slacklines at sunset above big waves in Nazare, Portugal on December 27, 2017. (Photo by Aidan Williams/National Geographic Travel Photographer of the Year Contest)

In the mountains of central Vietnam, a colossal pair of hands lifts a golden thread of walkway high above the clifftops, as if the mountain itself has sprouted limbs. Here: A giant hand structure at the Gold Bridge is seen on Ba Na hill near Danang city, Vietnam on August 1, 2018. (Photo by Reuters/Kham)

Sheep stand on a dyke as clouds cover the sky over Tossens, northern Germany, on September 2, 2019. (Photo by Mohssen Assanimoghaddam/dpa/AFP Photo)