Cleveland Metroparks naturalist Carly Martin was off-trail with a group of schoolchildren, searching for salamanders in the South Chagrin Reservation. For most of the year, salamanders hide beneath the leaf litter or under rotten logs on the forest floor. Digging through this organic detritus, one of the children discovered something that was clearly not an amphibian.

It was a clear plastic container, with the word "geocache" written on one side, but that meant nothing to Martin at the time. The contents shed little light on the purpose of the container - inside was a small notebook, pencil, some Matchbox cars and other assorted trinkets. "I figured it was some sort of scavenger hunt, and I told them to put it back so we wouldn't ruin the game. I meant to go back and look at it more closely, but I forgot about it."

Months later, her supervisor asked Martin if she'd ever heard of a hobby known as geocaching. The question jogged her memory. "I said no, I've never heard of it - but I had found one."

Since then, Martin has become the Metroparks' resident expert on geocaching, a fast-growing pastime that involves a treasure hunt guided by handheld personal navigation units, commonly known as GPS receivers. Geocache enthusiasts hide small treasure troves in secret locations, and then post the longitude and latitude coordinates, known as waypoints, on the Web. Others find the treasure, record their visit in a logbook, and then report their hunt results online. The activity relies on the Global Positioning System, a worldwide radio-navigation system developed by the U.S. Department of Defense. The system uses multiple signals from orbiting satellites to pinpoint exact longitude and latitude.

The Cleveland Metroparks could have reacted with alarm to the presence of unsanctioned treasure hunts on park property. After all, urban park systems abhor unguided off-trail activity. But instead of launching a search-and-destroy mission, Martin and her bosses also saw an opportunity in geocaching to bring a new group of tech-savvy visitors to the parks - people who might otherwise spend a sunny summer afternoon glued to their computer screens.

Pretty quickly, Martin discovered this was a movement with some legs. The most popular Web site for this activity - www.geocaching.com - lists more than 900 caches hidden within a 25-mile radius of the South Chagrin Reservation. Amazon.com offers at least a half dozen geocaching guide books, all published within the last two years.

"I like sharing nature with people as an experience, and geocaching fits in well with that," says Martin, who has in recent years answered phone calls about geocaching from park systems across the country. "Rather than get rid of them, we decided to embrace them."

And gain some control over them. People who want to hide a cache on Metroparks property must now get a permit. The site is reviewed to make sure it is not in an ecologically sensitive area, and Martin allows off-trail caches to stay in one place for no longer than a year, to limit damage to the undergrowth.

In addition, the Metroparks' staff hide about five caches each year at various parks, in artificial rocks and logs stuffed with trinkets carrying the park system's logo. They also host an annual "Cache-athlon" and picnic, which draws about 150 "cachers" each year. Altogether, based on records on the geocaching Web site, Martin estimates that about 300 to 400 people are finding caches each year on park property.
Geocache enthusiasts cite various reasons for enjoying the hobby. Some are techno-geeks, others just love the outdoors and the thrill of the hunt.

"It gives you a goal when you're hiking. You get to go out and see nature, but you also have an end result," explains Keri Aller, a physical therapist from Sylvania. "It gives us a route for exploring."

Aller's husband, Bill, bought a GPS unit about five years ago, mostly to navigate his way back to his truck after riding his dirt bike in remote areas. But after a co-worker introduced her to geocaching, Keri found the GPS could serve the entire family.

With the help of their GPS and their Labrador retriever, the couple has located 34 to 40 caches, mostly in Ohio.

"I've been camping all over the state, but through geocaching, I've been to places I didn't even know existed," Keri Aller says. "In the Hocking Hills, one is hidden in an old abandoned railroad tunnel. You're walking around looking for this thing in the complete dark. It gets kind of spooky, but it's fun."

They've located most of their sites in Ohio, but they've also found them while camping in Pennsylvania, at historic sites in downtown Pittsburgh, and in Colorado and Wyoming.

The rules are simple. "If you take something, you leave something," Aller says. There are variations on the theme. "Travel bugs" are intended to be carried from one cache to another, and registered users can track their progress on the Web.

And watch out for the muggles, the people who don't know about geocaching. (The term was stolen from the Harry Potter series of novels, where it is used by the wizard children to describe non-magical folk.) If muggles find the cache they might intentionally plunder it, or worse, move it to a location far from the published waypoints, never to be found again.

"If you're in a park and there are a lot of people around, you have to be discreet," Aller explains.

Martin says the spy-vs-spy aura about geocaching is appealing to many. Cachers use code names on the Web site, like Funky Monkey and the Hiking Viking. And they often couch their descriptions in code ("between the earth and the sky, but not in a tree").

Geocaching offers a variety of options, with caches rated on a five-star scale based on the difficulty of the terrain and the cleverness of the

"hide." There are caches that take five minutes and can be found a few feet from your car - and there are caches that require mountaineering equipment and are for experienced climbers only. You can find a cache on your lunch hour, or you can build a vacation around them.

There are caches based on hiking through beautiful scenery, and some that promote local history. In the latter category, many are "virtual caches" that do not involve a hidden container. In Sylvania, for example, published waypoints lead participants to the cemetery where the original author of the Nancy Drew book series is buried. To get credit for finding this one, you simply e-mail the "hider" with the answer to a question that can be found only in the cemetery.

Early attempts might not pan out. On a gentle autumn evening recently, my three adult daughters and I attempted to find a cache at a historic landmark in Nelsonville. The cache may have been there, but we couldn't find it. After 30 minutes of searching an area, we went back to the Web listing for this site, and discovered comments from previous searchers that suggested the cache might have been plundered.

The next morning, we walked through a heavy fog but had to give up when our GPS unit lost its signal beneath the dense forest canopy,
"Geocaching isn't meant to be easy," Martin explained, when I e-mailed her, asking for tips. "The GPS gets you close, but you have to look for other clues as well."

Later, after more practice with the GPS, we found two caches hidden in separate spots in the same park located along the Olentangy River in Columbus.

The first was designated as easy, both in terms of terrain and the hiding place. And it was. We were directed to the scrubby wooded area along a dry creek bed - the kind of place where you might be surprised by what you find.

Finding the second cache was more difficult, although the GPS unit pointed us in the right direction. First, we followed the west bank of the river, but as the river turned, the pointer directed us to the opposite bank. Not wanting to swim, we were forced to backtrack to a bridge crossing. Next, we were led off the easy bike path and into a confusing thicket of fallen trees, muddy paths and gullies leading to the river.

Despite a dozen side trails and dead-end options, the GPS led us to a rocky shoreline. We finally came upon several rocks that bore signs of recent scrapes.

"I see it, I see it!" my 7-year-old shrieked, after catching a glimpse of a red plastic lid wedged beneath some rocks. Inside were toy cars, a buckeye and an inexpensive plastic whistle with a compass mounted on one end.

We left a Happy Meal toy and took the whistle-compass - but we followed the GPS to find our way back to the car.

Getting Started Tips for Joining the geocaching crowd. More information about geocaching can be found on the activity's best-known Web site, www.geocaching.com. Free registration is required to access the waypoints and to post information - although a paid, "premium" membership also is available. Others sites have cropped up over the years, including a popular alternative site at www.navicache.com. Geocaching does not require an expensive handheld navigation device. Mariners, explorers and hikers have been using navigational waypoints for centuries. The modern precursor of geocaching - an activity known as orienteering - has been a popular outdoor activity for decades. In orienteering, participants use a United States Geological Survey map and compass to find hidden markers based on the same longitude and latitude lines used in geocaching. But orienteering requires more-advanced skills in map and compass use. Geocaching owes its popularity to the increasing availability of handheld GPS units and the decision, in 2000, to unscramble the signals of the military navigational satellites, opening the door to civilian use. GPS receivers, many no larger than a cell phone, can be purchased in stores or online at any number of outdoor outfitters. We used a $99 Garmin eTrex to get us started, and it was all we needed. Less-expensive models are available, and more bells and whistles can be found on more expensive units. But even the best GPS won't find the cache alone. The handheld units generally have a margin of error of about 30 feet, and cache hiders can be remarkably clever. "It [your GPS unit] should take you to within a strong stone's throw," explains Carly Martin, a Cleveland Metroparks naturalist who coordinates geocaching at the parks. If you are going to hide a cache, make sure you check with the property owner, whether the site is in a public park or on private land. The geocaching community discourages trespassing, and the managers of geocaching.com will block information about a cache if they are notified that it has been placed without permission.