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Spelunkers understand that their journeys often lead to the discovery of awe-inspiring natural wonders, hidden away in the damp, shadowy recesses of caves. Forces such as water, wind, minerals, temperature, and pressure combine to create breathtaking geological and ice formations.
10. Cave Bacon
A widely admired and striking cave formation is the cave drapery. This formation occurs when water, rich in calcite, clings to a slanted ceiling, held in place by surface tension. As the water trickles downwards, it releases carbon dioxide into the air, making the solution rich in calcite, which solidifies into a thin strand. Additional droplets follow the same path, adding more layers over time. Eventually, these layers form delicate, wafer-thin sheets of calcite resembling draperies hanging from the ceiling.
Small undulations in the cave's ceiling create ripples in the drapery, which are often enhanced into folds and curls, especially along the lower edges. Occasionally, the calcite-rich water contains iron oxide or organic materials, coloring the drapery in shades of yellow, brown, and orange. As the concentrations of these materials vary in the droplets, they leave behind colorful bands, making the draperies resemble strips of bacon hanging from the ceiling. Almost every cave around the globe has draperies, and many also feature cave bacon.
9. Cave Balloons
Cave balloons are so uncommon that little is known about their formation. It is believed they occur when high-pressure solutions seep through the tiny pores in cave walls coated with moonmilk. Moonmilk is a widespread surface deposit in caves, formed by dripping water on the walls or ceilings. It consists of fine, white crystals, typically carbonates like calcite or hydromagnesite. When moist, moonmilk has a creamy, cheese-like consistency and is highly pliable. If moonmilk is composed of hydromagnesite, it may envelop the seeping solution, expanding like a balloon, often exhibiting a lustrous, pearl-like shine.
Cave balloons are temporary formations that eventually burst or dry out and crack, especially in caves with low humidity. The Jewel Cave near Custer, South Dakota, is one of the best-known locations to find them. They have also been discovered in the caves of the Guadalupe Mountains in New Mexico, particularly in the Carlsbad Caverns.
8. Cave Flowers
Cave flowers are primarily composed of sulfates, usually gypsum. When water absorbs carbon dioxide and sulfur and then drips through the cave walls, it can evaporate, leaving gypsum crystals behind. Unlike stalactites, which grow from the tip downward, cave flowers grow outward from the point where the water originated, forming curved, petal-like structures. These curves are the result of fluctuations in the water flow rate. They are found in abundance in Mammoth Cave, Kentucky. In 1896, a geologist described the blossoms he encountered there:
From a central stem, countless fibrous and transparent crystals curl gracefully; each tiny crystal is a miniature study; every cluster of curved prisms is a marvel; and the entire blossom is a breathtaking work of beauty. [. . .] Floral clusters, bouquets, garlands, and wreaths embellish nearly every inch of the ceiling and walls. [. . .] There are clumps of lilies, pale pansies, blanched tulips, drooping fuchsias, sprays of asters, spikes of tube-roses, and wax-leaved magnolias. [. . .] The imagination finds every greenhouse gem and garden delight in this crystalline wonderland.
The blossoms described above can be seen around the 18:20 mark in the video above. Cave flowers are also prevalent in Lechuguilla Cave, Carlsbad Caverns National Park, New Mexico.
7. Frostwork
Frostwork can be made of opal, gypsum, or even ice, but it is most commonly composed of aragonite. Though it is usually white, it can also appear in various colors. Aragonite crystals have a distinct, needle-like shape, and over time, they develop into frostwork formations that resemble “bushes.”
Aragonite frostwork is a rare chemical phenomenon because aragonite is not stable in the cave’s low-pressure, low-temperature environment. Typically, calcium carbonate solutions form calcite in these conditions. However, when magnesium ions are present, calcite formation is blocked, causing the solution to become supersaturated and form needle-like aragonite as it evaporates. This is why frostwork often emerges from calcite formations such as popcorn nodules or stalactites.
Frostwork is relatively common, but its delicate beauty often makes it a target for vandalism. As a result, it is best appreciated in caves that are either protected from human interference or rarely visited. You can find frostwork in caves across Australia, South Africa, Korea, Japan, Romania, Norway, Brazil, and the United States.
6. Shelfstone
Shelfstone is a flat, typically smooth ledge or pedestal that resembles a coffee table with a glass top. It usually forms when calcite precipitates on the surface of a cave pool. These precipitates often float, forming ultrathin rafts. As additional calcite precipitates, the shelf extends laterally just beneath the water’s surface until it attaches to a wall or formation, becoming a shelf. It can also grow downward, sometimes reaching all the way to the pool’s bottom, forming tables or lily pads.
The result is a formation that can be as thin as paper or thick enough to support the weight of a person. If the water level later drops, multiple layers of shelfstone can form in the same area, sometimes in varying colors. Some of the most spectacular shelfstone can be found in Lechuguilla Cave.
5. Boxwork
Boxwork derives its name from its resemblance to the corrugated surface of a cardboard box. Unlike speleothems, which form through precipitation or evaporation, boxwork is a speleogen created by erosion or dissolution of the surrounding rock.
Essentially, it is a network of intersecting veins or fins, each as thin as paper. These veins are made of calcite crystals held together by a quartz matrix, and they fill the cracks and seams in the bedrock ceiling or walls of the cave. The calcite matrix is more resistant to erosion and dissolving agents than the bedrock, allowing it to remain intact while the surrounding rock erodes or dissolves.
The oxidation of metallic sulfides gives boxwork its characteristic ocher, orange, or dark brown colors. Boxwork appears in triangular, pyramidal, diamond, and irregular shapes. It is rare, typically forming in dry caves where there is no dripping water. Some of the best examples of boxwork are found in Wind Cave, located in the Black Hills of South Dakota.
4. Cave Pearls
Cave pearls develop in shallow pools when water droplets saturated with calcite land. As the carbon dioxide is removed from the droplet, calcite precipitates and coats small debris resting on the pool’s floor. This debris can range from sand to bones or even parts of other cave formations, like rafts or soda straws. With each subsequent coating of calcite, the pearl’s size can increase by as much as 20 centimeters (8 inches). Even if the original debris is irregularly shaped, the pearl typically forms into a sphere, as this shape has the least surface area for the most efficient calcite deposition. However, pearls can also take on elliptical, cylindrical, or even cubical forms.
Occasionally, extra calcite forms a cup or nest around the pearl, allowing it to sit within. Agitation of the pool polishes the pearl into a lustrous white shine. This movement also prevents the pearl from sticking to the pool's bottom until it grows large enough that even agitation can’t shift it. At that point, it becomes cemented to the rock beneath. Cave pearls are abundant, with literally millions found in Gruta de las Canicas, a cave system near Tabasco, Mexico.
3. Giant Selenite Crystals
Previously, we discussed the giant selenite crystals found in the Naica mine near Chihuahua, Mexico. What is truly remarkable is the process behind their formation. While most caves are cool, the Naica mine is situated above a magma intrusion, causing the temperatures in the Cave of Crystals to soar to 44 degrees Celsius (112 °F), with 100% humidity. Visitors to this awe-inspiring location must wear respirators and special suits with ice packs sewn into the upper part of the body. Even with these precautions, tours are limited to just 20 minutes. The Cave of Crystals was discovered in 2000.
For centuries, water rich in calcium sulfate poured into these caves. When the temperature eventually stabilized at 58 degrees Celsius (136 °F), minerals in the water—especially gypsum—transformed into selenite crystals. The enduring, steady temperature allowed the crystals to grow to unparalleled sizes. The largest crystals reach lengths of 11 meters (36 feet), have diameters of 4 meters (13 feet), and weigh up to 55 tons. Despite their enormous size, these giant crystals are soft and easily scratched. It is estimated that the oldest crystals are over 600,000 years old.
2. Ice Caverns
Ice caves, also known as ice caverns, are primarily created by water, not geological forces. However, they are equally awe-inspiring, featuring frozen flows, ice columns, and stalactites. These ice caverns take many shapes. Sometimes, glaciers form caves when they hollow out. The Mendenhall Ice Caves, located beneath the glacier of the same name near Juneau, Alaska, are a prime example. The glacier stretches 19 kilometers (12 miles) in length, although it has been retreating, having lost nearly 3 kilometers (2 miles) since 1958.
Another type of ice cave forms when snow piles slide down the sides of mountains and accumulate at the base. These snow masses are then hollowed out by runoff. A well-known example is the Big Four Ice Caves in the Mount Baker-Snoqualmie National Forest in Washington. Tragically, in July 2015, one hiker was killed when the ceiling of one of these caves collapsed.
In other places, the combination of winter or a nearby glacier can create ice caverns at the entrance of a cave. For example, the first kilometer of the 42-kilometer (26-mile) Eisriesenwelt Ice Cave in Austria freezes each winter. As spring arrives, the ice near the entrance melts and seeps into the still-frozen lower sections, forming stunning ice formations that remain through the summer months. A similar ice cave can be found in Skaftafell, Iceland.
1. Helictites
Helictites, like most speleothems, are formations created by water enriched with calcite. They often begin as stalactites or soda straws hanging from the ceiling or walls of a cave. Water flows through a pore, down a narrow capillary or tube, and reaches the tip where calcite precipitates and forms the downward-growing structure.
At some point, something disrupts the water flow, diverting it in a new direction. This change in flow alters the capillary's path, resulting in a formation that seems to defy gravity. Such disruptions may be caused by impurities in the water, blockages in the original capillary, irregular crystal formations, or even air currents. The outcome is a variety of shapes, from delicate threads to branching antlers to elegant spirals. There’s even a unique type of helictite that grows underwater and resembles floating pasta. It can even branch out, creating multiple arms that evoke the chaos of a plumber’s nightmare.
Underwater helictites are found in Lechuguilla Cave in New Mexico. The stunning helictites in Black Chasm Cavern near Volcano, California, are so remarkable that the cave has been designated as a National Natural Landmark.
