I am in the Phong Nha-Ke Bàng National Park in Vietnam on a five-day expedition to the world’s largest cave.
In 2014, once the caves had been thoroughly explored, the government granted tour operators permission to host treks through the caves. Since then, approximately 900 licences have been awarded, and I got one of those – Lucky me!

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The main chamber is more than 5 kilometers (3.1 miles) long, 200m (660 ft) high and 150m (490 ft) wide. At this scale, a half-mile block of 40-story buildings could fit inside this lit stretch of Hang Son Doong cave.
The cave features two collapsed dolines, or sinkholes, that let daylight in. This has given life to the cave, making it possible for two separate forests to grow within it, a feature not normally associated with the subterranean recesses of caves.

The main cavern is so large that it has its own climate, and clouds have even been known to form within it.
The cave inside has some of the tallest known stalagmites at around 80 meters tall. Son Doong is a remarkable natural wonder in many respects.

More info: cave.photography