Being dark and narrow and perilous, slot canyons are fearsome places to us ordinary mortals. But to hard core canyoneering enthusiasts, a slot canyon is a place for fun, and the steepest, narrowest, darkest, and most dangerous ones around are irresistible.

Zion National Park is the most magnetic of all the canyoneering hotspots in the U.S. More than four dozen challenging slot canyons feed into Zion Canyon, a 15-mile long gorge that the Virgin River has carved into the Navajo Sandstone in this marvelous park. Nearly all of these tributary slots are remote and technically difficult. Many are awesome to contemplate and all are beautiful to behold.

The canyons deserve to be called slots, for they are indeed remarkably narrow. In various places where nearly sheer walls soar hundreds of feet toward the sky, the canyon bottoms are almost astonishingly constricted. The gold standard slot is one so narrow that a canyoneer can stick out his arms and touch both walls.

Many slot canyons that canyoneers find appealing have steep descents, no-return rapells, waterfalls, log-filled pools, slick rock passages, talus tunnels, lengthy wading passages, and other challenges going far beyond those encountered in more “tame” canyons that attract canyon hikers.

While simple canyon hiking can be a moderately strenuous activity for day-trippers and weekenders, true canyoneering (more often called “canyoning” outside the U.S.) is an extreme sport that involves rough-terrain hiking, rock climbing, rapelling, cold water and strong current swimming, jumping that is more than routinely difficult, and miscellaneous other wilderness technical and survival skills such as navigation and route-finding in complicated terrain. And don’t even think about canyoneering if you are claustrophobic, afraid of snakes, or cannot bear the thought of being injured while far from the nearest source of help.

All canyoneers know the story of Aron Ralston, a young man who saved his life by using a pen knife to amputate his left arm below the elbow when it was crushed and pinned by a loosened boulder while he was soloing a slot canyon near the Maze District of Canyonlands National Park in April 2003.

Aron, who described his experiences in his book Between a Rock and a Hard Place, is a resilient guy. Now wearing a prosthetic device, he still climbs every chance he gets. Presumably, he is now more inclined to let somebody know where he is going and when he expects to return.

Practitioners of the rapidly growing sport of canyoneering are keenly aware that slot canyons are no place to be when flash floods sweep through them. It is these sudden, raging torrents and their scouring debris that have done most of the stone carving in these narrow canyons. They are implacable killers, and only somebody with a serious death wish would knowingly be in a slot canyon when a flash flood roars through.

A video clip showing a flash flood in a slot canyon at Escalante, Utah can be seen at Flash Flood! Canyoneering in Escalante, Utah. Be sure to also watch Flash Flood Part #2, which provides better views of the slot canyon..

Canyoneering is growing by leaps and bounds. At Zion National Park, it is now so popular that permits are required and daily quotas are enforced.

Michael Ybarra is the Wall Street Journal’s extreme sports correspondent. (How many people would guess that the venerable WSJ has an extreme sports correspondent?) Here is how Michael described a recent experience in Zion as a neophyte canyoneer:

The narrow canyon turns wide; the sun reappears ; trees, bushes, and birds materialize. Two small rattlesnakes slide away toward the shade. Logs bigger than telephone poles litter the bottom of the gorge, freshly splintered like toothpicks – a reminder of the fury that flash floods can release in this tight space. We venture through a tunnel that tumbles into a stagnant smelling pool of water…..

If prose like that makes you want to want to drop what you're doing, pack your gear, and head for the nearest slot canyon, you just might have the makings of a canyoneer.

If you’re serious, as in seriously interested in staying alive and healthy, you’ll want to make your initial canyoneering forays with people who really know what they are doing. You might also want to take advantage of the canyoneering courses, publications, and guidance offered by the American Canyoneering Association, which not only promotes proper training, but also espouses responsible wilderness ethics.

The latter consideration is important, because canyoneering is an emerging managerial challenge in our national parks. Park managers are becoming increasingly concerned about the potential impacts of this relatively new extreme sport in wilderness areas. The technical difficulties of conducting search and rescue operations in slot canyons have also drawn attention.