Vietnam’s Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle. Despite its size, Son Doong wasn’t discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009. (Photo by Jason Speth/HuffPost)

Fire fighters, emergency service vehicles and a helicopter in Moskovsky Prospekt at the entrance to Tekhnologichesky Institut station of the St Petersburg metro in the aftermath of an explosion which occurred in a train at 14:40 Moscow timeon April 3, 2017. (Photo by Peter Kovalev/TASS)

People wear clothing to protect them from the heat while riding along a street in Hanoi, Vietnam, 24 April 2019. North and central Vietnam was recently affected by extreme hot weather, with the highest temperatures reaching over 40 degrees Celcius, according to the National Centre for Hydro-meteorological Forecasting. (Photo by Luong Thai Linh/EPA/EFE)

Boston Bruins center Charlie Coyle (13) goes airborne as New York Islanders goaltender Semyon Varlamov (40) makes the save in a shootout during an NHL hockey game, Tuesday, December 13, 2022, in Boston. (Photo by Charles Krupa/AP Photo)

Three men display their tattoos during the International Malaysia Tattoo Expo in Kuala Lumpur on November 29, 2019. The international tattoo expo is held for the first ime in Kuala Lumpur celebrating that art of tattoo and gathering over a hundred tattoo artists from 40 countries. (Photo by Mohd Rasfan/AFP Photo)

A mural painted on a fence depicts US President Donald Trump as the Coronavirus in San Francisco, California on April, 01, 2020. All 40 million residents of California were Thursday ordered to stay at home indefinitely in a bid to battle the coronavirus pandemic in the nation's most populous state. (Photo by Josh Edelson/AFP Photo)

Children's author and charity fundraiser Chris Green, of Embsay, near Skipton, has been dubbed “Rhino Boy” as he has pledged to run 40 marathons and other events dressed as a Rhino to raise money for Save the Rhino on January 2020. (Photo by South West News Service/Yorkshire Post)

A participant competes in the “Nostalgic Ski Race” in the western town of Neuastenberg February 8, 2015. The “Nostalgic Ski Race” is held every two years with about 40 participants and is organized by the ski club of Neuastenberg, a town which was founded in 1713. The conditions for the participation in the race are vintage skis and dresses. (Photo by Ina Fassbender/Reuters)