A pilgrim prays at Mount Al-Noor, where Muslims believe Prophet Mohammad received the first words of the Koran through Gabriel in the Hera cave, ahead of the annual haj pilgrimage in the holy city of Mecca, Saudi Arabia September 7, 2016. (Photo by Ahmed Jadallah/Reuters)

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)

Aerial view of the “Viracocha III”, a boat made only from the totora reed, as it is being prepared to cross the Pacific from Chile to Australia on an expected six-month journey, La Paz, Bolivia, October 19, 2016. An expedition in a boat made only of reeds crafted by indigenous Bolivians is getting ready to cross the Pacific from South America to Australia, in a fresh attempt to prove that ancient mariners were capable of making the journey. (Photo by David Mercado/Reuters)

Francisca Gomez stands at an entrance to her home, a cave where she has been living for the past 50 years, in the mountains on the outskirts of Chusmuy, Honduras April 21, 2015. The 75-year old widow receives help from neighbors and friends and she makes a little money by selling wood she collects, according to a local newspaper. (Photo by Jorge Cabrera/Reuters)

A man works on “Long Ma”, a creation by La Machine production company, during a media visit to “A Journey to Nantes” (Le Voyage a Nantes) art festival in Nantes, France, June 30, 2015. The art festival will run from July 3 to August 30. (Photo by Stephane Mahe/Reuters)

An unidentified man brandishes a gun during a protest against a new citizenship law outside the Jamia Millia Islamia University in New Delhi, India, January 30, 2020. The gunman went live on Facebook to warn he was taking his “final journey” before firing at the protest. (Photo by Danish Siddiqui/Reuters)

A supporter welcomes US army convoy as it enters Czech Republic in Harrachov, Sunday, March 29, 2015. The “Dragoon Ride” convoy started last week from Estonia and passed through Latvia, Lithuania and Poland before entering the Czech Republic on a return journey to a German base. (Photo by Petr David Josek/AP Photo)

Thousands of heavy-duty trucks loaded with coal are lined up for up to 130 kilometres from the Mongolia-China border on a sole road in the Gobi desert, Mongolia, October 29, 2017. The journey can take more than a week. (Photo by Bazarsukh Rentsendorj/Reuters)