Sơn Đoòng cave in Phong Nha-Kẻ Bàng National Park, Vietnam

© David A Knight/Shutterstoc

An underground universe

By far the world’s largest cave by volume, Sơn Đoòng in Vietnam is big enough to fly a Boeing 747 through. The stalagmites rising from its floor stand 15 storeys high, its main chamber is 650ft (198m) tall, about 500ft (152m) wide and stretches for more than three miles. Inside one of its massive dolines - sunken, sun-exposed sections like that seen in our image - is an entire rainforest ecosystem, and that’s just what we know so far.

Vietnamese farmer Hồ Khanh first stumbled upon this cave in 1991, discovering a huge, dark, untouched chamber, complete with a free-flowing underground river. Lacking the gear to explore the cave further, Khanh wandered back home to tell friends about his discovery. But he was unable to retrace his steps to the cavern and it would remain unexplored for almost two decades, until a team of British cavers caught wind of Khanh's tale.

In 2009, Khanh led them to Sơn Đoòng after a long search, and they discovered that the cave was vast beyond anyone's expectations. 11 years later, we still don’t know it all: Cave divers have yet to fully explore Sơn Đoòng's subterranean river, and more vast passages were discovered as recently as 2019. This enormous underground world is home to many secrets, yet to be revealed.