Carlsbad Caverns National Park - The Action

When rain falls and the drops seep through the soil, they combine with carbon dioxide to form a weak carbonic acid. Trickling down through cracks in the reef, this mild acid is strong enough to dissolve limestone, generally in the form of calcite, which it carries along. When a drop of this stone-in-solution reaches the ceiling of an underground cavern, some of the carbon dioxide returns to the air, and crystals of limestone are released. More drops follow the same path, leaving their limestone loads, until a new rock formation, a stalactite, hangs from the roof.

Generally, the drop deposits a ring before it falls. Another drop adds its ring, and then another, building a tubular stalactite called a soda straw, through which drops of water run. This process may continue until, like hundreds of examples hanging from the level ceiling of Carlsbad's Soda Straw Forest, the tubes are several feet long. But when a prospective soda straw becomes plugged, probably because a droplet evaporated before it could fall, water starts to run down the outside of the formation, making a solid stalactite, usually in the familiar icicle shape.

Or the tube may become so constricted by lime deposits that drops no longer flow freely but are forced out by capillary action. The resultant helictite may seem to defy gravity, growing first to one side, then the other, even growing upward and then downward again. Not all helictites begin as soda straws: some arise from the floors of caves, some sprout from the walls, some grow from existing structures. There are many complex theories to explain why helictites behave as they do. Though none of them is entirely satisfactory, it seems certain that the irregular shapes of calcite crystals have a lot to do with it. Similarly, delicate epsomite needles owe their flimsy, hairlike forms to the needle-shaped epsomite crystals of which they are made. In Carlsbad Cavern some get to be 18 inches long, but this is exceptional: most break of their own weight first.

Sometimes water runs across the floor to form an "icing on the cake" called flowstone. Or a single droplet may run along a ridge or down a sloping wall, leaving a winding trail of stone behind. A million more droplets follow the trail, and a flowstone drapery is created, rock as soft-looking as velvet, or perhaps it becomes slabs of the intriguingly named cave bacon, hanging from the roof of a passage like meat on a hook.

Falling drops of calcite-laden water also leave their deposits on the cavern floor, and as they pile up, a stalagmite is born. Slowly, it grows upward toward the source of its substance. When a stalactite and a stalagmite meet, they form a column. Intricate layers of delicate dripstone may adorn its surface.

When drops fall onto limestone quickly enough and with enough force, they may carve out a nest, and then fill the nest with cave pearls. These opalescent balls form around tiny pebbles of grains of sand in puddles of heavily saturated water no more than an inch or so deep. Most are round; some, forming around bat bones or bits of debris, may be of any shape imaginable.

These forms are common to limestone caves all over the world. Indeed, some caves hold examples of one or another formation that Carlsbad cannot match. But in no cave yet discovered, a necessary qualification when speaking of caves, as people stumble regularly into new underground wonders, does such a variety of ornamentation come together with such symphonic beauty. In part this is because the easily dissolved limestone reef, with its myriad cracks and fissures, is ideal building material. In part, too, it is because of the way the climate in this region has changed over the ages since the Guadalupe Mountains arose.

A change of climate in the world above causes a change in the amount of water seeping down, and probably in the routes the water follows. It can also cause a change in the acid content of each drop: the less moisture in the soil, the less life; and because carbon dioxide is a by-product of the life process, less life means less carbon dioxide to form carbonic acid. Plentiful water, heavily saturated with calcite and running through clearly defined channels, can make for massive stalactites, stalagmites, and columns. Carlsbad has many of these, right up to the 60-foot Goliaths in the Hall of the Giants. A less plentiful supply might mean that stalactites grow long and slender, with no matching stalagmites, because droplets evaporate before they can fall. Other variations make for flowstone draperies, such as the graceful silken stone that cloaks the walls and throne of the Queen's Chamber.

Rarely, the stone-in-solution forms rounded, hollow shapes known as hydromagnesite balloons, they puffy form determined by the minerals mixed into the calcite. These structures may be a foot or more across. Carlsbad Cavern is one of only three caves in the United States where hydromagnesite balloons are found.

Gleaming in the light of a lantern like intricately cut diamonds, aragonite trees are probably the most beautiful of Carlsbad's strange formations. Like hilictites, they may grow from walls, floors, ceilings, stalagmites, anywhere, and their spiny branches subdivide and twist and turn in all directions. When you come upon one of them unexpectedly, it is easy to see why they were once thought to be alive, plants made of stone.

Other minerals paint the normally beige or milk-white formations with a wide range of color. A structure may have been freshly washed (say, in the last 10,000 years) with the red of iron ore, streaked with the purple and blue of manganese oxide, or blotched and spangled with almost any other hue. Or the colors may be deep within, beneath recent layers of glossy white. Many dark, muddy-looking columns and stalagmites have been adorned with pigmented flowstone, then hung with jagged dripstone that looks like fine bone china, and finally crowned with crystalline aragonite jewelry.