Son Doong ("Mountain river") cave in Vietnam is the largest cave in the world. It was discovered first in 1991 by a local farmer, then in 2009 British explorers relocated it with the farmer's help. About 9km (5.5mi) long, with a rushing river and caverns that could hold an entire New York City block with 40-floor skyscrapers, it's more than twice the size of the largest previously known cave, Deer Cave in Malaysia. Just imagine—a Boeing 747 jet plane could fly through some areas without the wingtips touching either side!
Even more incredible is the rainforest that has grown beneath a place where the limestone ceiling collapsed. Vegetation, insects, birds, and other animals (including tigers!) all live in this miniature forest. Elsewhere, there are "cave pearls" the size of baseballs, stalagmites 70m (230ft) high, and even a sandy beach.
In 2019, an underwater tunnel was discovered linking Son Doong cave with another nearby cave called Hang Thung, which adds another 1.6 million cubic meters to Son Doong's 38.5 million. With the additional space, any other cave currently known in the world could fit inside.