A veil of darkness cloaks the natural beauty of caves. Some are found in cliffs at the edge of the coastline, chipped away by the relentless pounding of waves. Others form where a lava tube’s outer surface cools and hardens and the inside of the molten rock drains away. Caves even form in glaciers where meltwater carves tunnels at the beginning of its journey to the sea.
But most caves form in karst, a type of landscape made of limestone, dolomite, and gypsum rocks that slowly dissolve in the presence of water with a slightly acidic tinge. Rain mixes with carbon dioxide in the atmosphere as it falls to the ground and then picks up more of the gas as it seeps into the soil. The combination is a weak acidic solution that dissolves calcite, the main mineral of karst rocks.
The acidic water percolates down into the Earth through cracks and fractures and creates a network of passages like an underground plumbing system. The passages widen as more water seeps down, allowing even more water to flow through them. Eventually, some of the passages become large enough to earn the distinction of cave. Most of these solutional caves require more than 100,000 years to widen large enough to hold a human.
The water courses down through the Earth until it reaches the zone where the rocks are completely saturated with water. Here, masses of water continually slosh to and fro, explaining why many caverns lay nearly horizontal.
Hidden in the darkness of caves, rock formations called speleothems droop from the ceilings like icicles, emerge from the floor like mushrooms, and cover the sides like sheets of a waterfall. Speleothems form as the carbon dioxide in the acidic water escapes in the airiness of the cave and the dissolved calcite hardens once again.
The icicle-shaped formations are called stalactites and form as water drips from the cave roof. Stalagmites grow up from the floor, usually from the water that drips off the end of stalactites. Columns form where stalactites and stalagmites join. Sheets of calcite growths on cave walls and floor are called flowstones. Other stalactites take the form of draperies and soda straws. Twisty shapes called helictites warp in all directions from the ceiling, walls, and floor. Source Article & photography by Carsten Peter/National Geographic Image Collection.
A giant cave column swagged in flowstone towers over explorers swimming through the depths of Hang Ken, one of 20 new caves discovered last year in Vietnam.
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 Hang 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. As “Sweeny” Sewell climbs to the surface, hikers struggle through the wryly named Garden of Edam.
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 Hang 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 author Mark Jenkins at the forest-shrouded entrance to Hang Son Doong. “Even though these caves are huge, they’re practically invisible until you’re right in front of them,” Jenkins says. Hunters have found caves by spotting winds gusting from underground openings.
Rare cave pearls fill dried-out terrace pools near the Garden of Edam in Hang 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, expedition organizers Deb and Howard Limbert 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 Hang Son Doong. Climbing specialists “Sweeny” Sewell and Howard Clarke here work on anchoring bolts to the slippery, porous rock to support the weight of climbers using ropes. Once over the wall, the expedition team discovered a second entrance into the cave.
Great Wall of Vietnam, a 200-foot cliff halted the advance of the first team to enter Hang Son Doong, in 2009. When explorers returned, Sewell drilled bolts for climbers to scale the obstacle with ropes. A white streak below, to his right, marks how high water rises during the wet season.
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 Hang Loong Con. A survey party discovered the cave in 2010, hoping it would connect with the enormous Hang Son Doong. A wall of boulders soon blocked the way, but a powerful draft indicated that a large cavern lay on the other side.
Enjoy more of Carsten Peter’s amazing cave photography as he ventures into Mexico’s Cavern of Crystal Giants, where massive beams of selenite dwarf human explorers deep below the Chihuahuan Desert.
Carsten Peter/National Geographic Image Collection