A climber ascends a shaft of light in Loong Con, where humidity
rises into cool air and forms clouds inside the cave.

A half-mile block of 40-story buildings could fit inside this lit stretch of Son Doong,
which may be the world’s biggest subterranean passage.

A jungle inside a cave? A roof collapse long ago in Hang Son Doong let in light; plants thickly followed.

Mist sweeps past the hills of Phong Nha-Ke Bang National Park, its 330 square miles set
aside in 2001 to protect one of Asia’s largest cave systems.

Going underground, expedition members enter Hang En, a cave tunneled out by the Rao Thuong River.
Dwindling to a series of ponds during the dry months, the river can rise almost 300 feet during
the flood season, covering the rocks where cavers stand.

Headroom shrinks in the middle of Hang En as cavers pass beneath a ceiling scalloped
by eons of floodwater rushing past. The river shortly reemerges onto the surface,
then burrows into Son Doong after a few miles.

Like a petrified waterfall, a cascade of fluted limestone, greened by algae,
stops awestruck cavers in their tracks. They’re near the exit of Hang En.

Moss-slick boulders and a 30-foot drop test adventurers at the forest-shrouded entrance to Son Doong.

Son Doong’s airy chambers sprout life where light enters from above
—a different world from the bare, cramped, pitch-black spaces familiar to most cavers.
Ferns and other greenery colonize rimstone. In the jungles directly beneath
roof openings, explorers have seen monkeys, snakes, and birds.

Rare cave pearls fill dried-out terrace pools near the Garden of Edam in Son Doong.
This unusually large collection of stone spheres formed drip by drip
over the centuries as calcite crystals left behind by water layered
themselves around grains of sand, enlarging over time.

Navigating an algae-skinned maze, explorers lead the way across a sculpted
cavescape in Hang Son Doong. Ribs form as calcite-rich water overflows pools.

The trickiest challenge for the expedition team was to find a way
over the
Great Wall of Vietnam, an overhanging mass of flowstone that
blocked the way deep inside Son Doong.

Like a castle on a knoll, a rock formation shines beneath a skylight
in Son Doong. A storm had just filled the pool, signaling that
exploring season was coming to an end.

Dubbed the Great Wall of Vietnam, a 200-foot cliff halted the advance of the first team
to enter Son Doong, in 2009.

“It sounded like a roaring train,” said an explorer, describing
the noise a second before a waterfall exploded into Son Doong
through the Watch Out for Dinosaurs doline, or sinkhole opening.

In the dry season, from November to April, a caver can safely explore Hang Ken, with its shallow pools.
Come the monsoon, the underground river swells and floods the passages, making the cave impassable.

Taking the only way in, a climber descends 225 feet by rope into Loong Con.
A survey party discovered the cave in 2010, hoping it would connect with
the enormous Son Doong. A wall of boulders soon blocked the way,
but a powerful draft indicated that a large cavern lay on the other side.

Streams of light from the surface unveil stalagmites fat and thin on the
floor of Loong Con. Cavers called the new find the Cactus Garden.