Watching as her father crawled up a muddy stream bank in Schenley Park, young Caitlin O'Connell remarked that Dad has been known to not only climb these hillsides, but also fall from them.

Once, while searching for hidden treasure on his hands and knees in North Park, Dave O'Connell encountered an icy patch, slid from a walking path and tumbled several yards to a cold creek below.

But these are the injuries to person and pride that geocachers are willing to endure.

Geocaching -- think treasure hunting with a global positioning system or GPS receiver -- is a growing hobby that combines a love for the outdoors and a love for, or at least a familiarity with, high-tech mapping equipment.

And, for youngsters like Caitlin O'Connell, there's the added thrill of finding hidden booty. Now in fifth grade, she was a second-grader when her teacher arrived at the lesson about sea pirates. She went home and announced to her father, "I want to hunt for treasure," and thus a family's addiction was born.

"It's a healthy addiction," Dave O'Connell said, as he and his 11-year-old danced over stones and ducked beneath fallen trees. "Once they figure out you're not a troublemaker, it's a pretty harmless activity."

When he says "they," O'Connell means state police troopers, park rangers, local bomb squads or anyone else who might wonder what a geocacher is doing along the side of a road, fiddling with a small box.

As the sport grows in popularity -- partly by word-of-mouth, partly because the cost of GPS units keeps dropping -- there have been more run-ins with state and local park officials who are initially uneasy about the idea of people hiding boxes, the sought-after "caches," in America's nature preserves.

In Colorado, for example, officials from Boulder County spent months studying and debating the potential impacts of the sport. The county is now monitoring, and limiting, the number of caches that may be hidden on public property.

The sport, said to have originated in Oregon, is 5 years old this month. To help celebrate, Magellan, one of the top companies selling GPS devices, is sponsoring a nationwide geocache search, starting today.

Geocaching has been slower to catch on in Pittsburgh than in some other places, possibly because of the terrain here, possibly because we prefer to watch somebody else taste the food before trying it ourselves. But popularity seems to be growing.

There are now maybe 100 serious geocachers in the Pittsburgh area, searching for the nearly 700 caches hidden within 50 miles of the Golden Triangle.

Getting started is easy. Somebody hides a cache, and logs the specific longitudinal and latitudinal crosshairs. He posts those coordinates on a Web site, such as geocaching.com, offers clues for novice hunters and the game begins.

Sometimes, the cache-keeper -- the person who originally hides it -- eschews the traditional box for alternatives, such as the multi-cache, wherein the first site offers a clue, directing the treasure hunter to another site, then another and so on.

Anybody with a handheld global positioning system receiver can play along.

Usually, the containers are filled with trinkets left by the treasure hunters -- earrings, toys, business cards, key chains -- and always, the containers hold a log book, which is supposed to be signed by all who find it.

If you take something, you leave something, and as long as you don't leave booze, tobacco or weapons, you're playing by the rules.

Oh, and if you're sleuthing in a public place, be on the lookout for "muggles" -- curious and unknowing onlookers, who at best will think you're a weirdo or at worst will call the police.

Sound easy? Well, sometimes it is. Sometimes it isn't. Sometimes the cache is just off a trail. Sometimes, it's in the middle of nowhere, up a steep cliff or in a deep ravine, not a trail in sight.

And sometimes, your GPS unit is useless.

"It isn't as easy as pointing an arrow," said O'Connell, of Sewickley.

GPS units figure out where they are by corresponding with satellites. But the receivers, while they can determine your location within 30 or so feet, are not exact. If your GPS unit is 30 feet off, and so was that of the guy who buried the treasure, you could be 60 feet from where you're supposed to be looking.

That's part of the fun, says Eric Milliron, of the South Side. His first geocaching trek, in February, took him and his girlfriend to Seldom Seen greenway in the Beechview section of Pittsburgh. He didn't find the Seldom Seen cache that day, and he still hasn't.

"It's very appropriately named," he said.

Milliron, 33, works for Allegheny County's economic development department. His new hobby is not only a weekend diversion, but also a way to get fellow enthusiasts to visit unfamiliar parts of the city.

"I've tried to use it as a community development tool," he said. For example, he's hidden a cache of his own in Fineview, which boasts spectacular, but little-known, views of the city skyline.

"The real excitement is exploring these unknown nooks and crannies," Milliron said before embarking on a planned geocache hike in Ohiopyle State Park. "I just like going places I've never been before, and exploring my own back yard."

The sport can be addictive. Many participants, like the O'Connells, plan vacations around their hobby. And while most participate recreationally, a few, like Mark Konopasek of O'Hara, do so competitively.

"It's the same thrill [as] going out and hunting an animal. ... I love 'first-to-finds,' " he said. That's the lingo for newly planted caches. "When a new one shows up, people might reroute their way to work."

That's because if you're the first to find the cache, you get online bragging rights among fellow hunters. Last year, Konopasek was the first to find a major cache hidden by Magellan in Schenley Park, he said.

Konopasek, who says he's been on 125 hunts, got hooked by O'Connell, whom he met two years ago at an Irish step dancing recital -- the daughters, not the fathers, are the dancers. O'Connell convinced him to give geocaching a try.

"I went on eBay, picked up a GPS," he said, and he's been hunting happily ever since.