In Deep: Go Underground to See the Best of Vietnam

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Carsten Peter / National Geographic / Getty Images

Vietnam hasn't made it easy for visitors to see the country's most fascinating superlative. At least 4 km long and more than 140 m high and wide in parts, Hang Son Doong is the world's largest known cave passage. But it lies in the heart of the Phong Nha-Ke Bang National Park on the Laotian border, reachable only after an arduous nine-hour trek and set in jungle so thick that many groups have had trouble finding the entrance or missed it completely.

Hang Son Doong was discovered by a local in 1991, but people from the area were deterred from venturing in because of the unnerving sound of fast-flowing water that can be heard at the cave entrance. (The noise originates from the 2.5-km-long underground river that gives Hang Son Doong, or Mountain River Cave, its name.) A British team made the first proper exploration in 2009, discovering stalagmites more than 70 m high.

If you're game, Phong Nha Discovery Tours,, runs trips to Hang Son Doong, from $450 to $900 per person and lasting two to three days. But the area offers around 150 limestone caves and grottoes, some of which are much more accessible. Phong Nha Cave, for example, is just a 15-minute boat ride from the national park's main entrance. Its dreamlike geological formations are enhanced by ancient Hindu altars carved into the rock. Boats can go about 1.5 km into this sanctuary, but if you're an expert caver, take along scuba gear: it's possible to venture another 3 km, but only after swimming through a black, underwater abyss.