One of my purposes in Underground Ranger is to show that some of nature's most astonishing creations are found below the earth's surface. The beautiful stalagmite pictured in the upper photo at left is at the bottom of an eighteen-story rappel in Ogle Cave, a part of Carlsbad Caverns National Park. It's made of calcite—limestone that's been dissolved and redeposited by water—and its pure-white color shows that it hasn't been contaminated by other dissolved minerals. Notice the crown of feathery aragonite cascading down the right side. Aragonite is a form of calcite that often crystalizes when water evaporates. In this case, the crystals point toward the right because that's the direction of airflow.

The gypsum chandeliers in Lechuguilla Cave's Chandelier Ballroom are the most spectacular formations in Carlsbad Caverns National Park. There are dozens of them, some up to twenty feet long, extending from the ceiling in several great clusters—long, white fingers of crystalline gypsum that curve in graceful arcs and terminate in bursts of selenite crystals and glistening gypsum spikes. To view the Chandelier Ballroom, go to en.wikipedia.org/wiki/Lechuguilla_Cave, then click on the image to enlarge it.

Other types of cave formations are shown at left and in the photo gallery, including cave popcorn and helictites, two of nature's most unusual creations. Underground Ranger explains how each of these wonders is created by the simple actions of water—just like the beauty of thunderstorms, the symmetry of rainbows, the shapes of dry washes and limestone cliffs, and the patterns of vegetation in a canyon. Water is nature's grand link between the extraordinary beauty of caves and the more familiar beauty we see around us on the earth's surface.

It's easy to remember where stalactites and stalagmites grow: the word "stalactite" contains a "c" for ceiling, while the word "stalagmite" has a "g" for ground. As a general rule, when water drips slowly and stays on a cave ceiling for a relatively long time, it tends to deposit most of its calcite there, creating a stalactite. When water drips rapidly and stays on the ceiling for only a short time, it tends to carry most of its calcite to the floor, where it may create a stalagmite. Depending on the drip rate and other factors, a stalactite may have a stalagmite growing directly below it, and if the two eventually meet, they form a column.

Helictites are an unusual result of something called hydrostatic pressure. Since limestone is porous, it absorbs water relatively easily, like a sponge, which means the limestone in a cave may contain varying amounts of absorbed water. If a tiny pore in the surface of the rock provides an outlet, the weight of the water—its hydrostatic pressure—can cause it to seep outward through the pore. It then begins to deposit calcite around the pore, and as more calcite accumulates it forms a small helictite with a tiny capillary tube running through its center. The water now seeps through the capillary tube toward the helictite's growing tip, and since gravity doesn't play a role in the helictite's creation, it's free to grow in any direction.