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)

A view of the start of the 37th Gotland Grand National motocross race in Visby, Sweden on October 24, 2020. (Photo by Soren Andersson/TT News Agency via Reuters)

Visitors look up at mock-ups of dinosaur skeletons inside the Museum of Natural History in Pyongyang on September 28, 2016. (Photo by Ed Jones/AFP Photo)

An emu runs to escape an approaching wildfire as it burns near Potero, California, U.S. June 20, 2016. (Photo by Mike Blake/Reuters)

A Russian peacekeeper shouts “No pictures!” at a checkpoint outside the city of Stepanakert on November 13, 2020, during a ceasefire in the military conflict between Armenia and Azerbaijan over the breakaway region of Nagorno-Karabakh. Russian began deploying 2,000 peacekeepers to Nagorno-Karabakh on November 10 after Armenia and Azerbaijan agreed a peace deal to end weeks of fierce fighting over the disputed region. (Photo by Alexander Nemenov/AFP Photo)

A pro-democracy protester wears a hat in the shape of a yellow rubber duck, which has become a recent symbol for the ongoing protests, during an anti-government rally in Bangkok on November 22, 2020. (Photo by Lillian Suwanrumpha/AFP Photo)

Exhausted grave diggers rest in between funerals at a cemetery designated for Covid-19 victims in Bandung on June 15, 2021, as infection numbers soar in Indonesia. (Photo by Timur Matahari/AFP Photo)

A Guarani Mbya man smokes a pipe next to a cut tree as he occupies land as a protest against real estate developer Tenda which plans to build apartment buildings here, next to his indigenous community's land in Sao Paulo, Brazil, Thursday, February 6, 2020. In response to an injunction filed by the builder, a judge has authorized the eviction of the indigenous protesters from the builder's property. (Photo by Andre Penner/AP Photo)