Hang En is the third-largest cave in the world by volume, though for most visitors the cave itself is almost a backdrop to the two-day trek required to reach it. The route cuts through jungle and involves several river crossings — you wade through streams, scramble over karst terrain, and arrive at the cave mouth to find a vast opening the size of a sports stadium. Inside, a sandy beach beside an underground river makes a campsite of unusual atmosphere.
The trip is operated exclusively by Oxalis and takes two days and one night, with camping inside the cave. It is far more accessible than Son Doong — physically moderate, affordable by comparison, and one of the best overnight cave experiences available anywhere. The sound of millions of swifts returning to roost at dusk is an experience in its own right.