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 row of abandoned oil tanks is filled with creepers on the riverside of Liyun River in Huai 'an, East China's Jiangsu Province, May 9, 2023. (Photo credit should read CFOTO/Future Publishing via Getty Images)

Maddie Stanton (L) and River compete in Helen Woodward Animal Center's 18th Annual Surf Dog Surf-A-Thon at Del Mar Dog Beach on September 10, 2023 in Del Mar, California. (Photo by Daniel Knighton/Getty Images)

Tourists fly in hot air balloons over the west bank of the Nile River on January 9, 2025 in Luxor, Egypt. Luxor, which contains the historical city of Thebes, capital of ancient Egypt's pharaohs at the height of their power, is full of pharaonic monuments and other antiquities. (Photo by Ahmad Hasaballah/Getty Images)

Stagecoach tests the amphibious bus, on the River Clyde on February 9, 2010 in Glasgow, Scotland. The Ј700,000 Dutch made amfibus could eventually replace the existing ferry service which is being scraped in March due to costs. (Photo by Jeff J Mitchell/Getty Images)

A North Korean soldier throws a stone towards a photographer on the banks of the Yalu River in the North Korean town of Sinuiju, opposite the Chinese border city of Dandong October 21, 2006 in Sinuiju, Democratic People's Republic of Korea. (Photo by Cancan Chu/Getty Images)

Seljalandsfoss waterfall, located in Iceland. Seljalandsfoss is one of the most famous waterfalls of Iceland. This waterfall of the river Seljalandsá drops 60 metres (200 ft) over the cliffs of the former coastline. It's possible to go behind the waterfall. (Photo by HotSpot Media)