By Lindsay J. Warner

For The Inquirer

They travel in packs, noses just inches away from GPS screens. Suddenly, "I found it!", one boy announces, grinning and peeling away from the group of a half dozen. The others intensify their focus. "Me too!", shouts another a moment later, until all six kids are crowded around a tree on Girard Avenue, pulling out from a knot a plastic screw-top bottle painted to resemble tree bark. It doesn't look like much from the outside, but everyone gathers around, eager to see what's inside.

This is geocaching (JEE-oh-cash-ing), a high-tech game played using coordinates and global positioning systems to find hidden "treasure." It's open to anyone - hiders or seekers - with a GPS and access to www.geocaching.com, where more than half a million registered users download and upload the coordinates of nearly 1 million hidden caches and relate their experiences from this worldwide scavenger hunt.

Although some adventures can take hours or even days, the contents inside the actual cache are rarely valuable - many times just a logbook and a small, worthless trinket.

But participants aren't in it for the goods. They say it's a great way to exercise. Or it brings them to otherwise off-the-beaten-track destinations or historical sites. Some consider it an extreme sport, pursuing geocaches hidden on mountain summits or in other nearly inaccessible locations. Others make it a practice to find everything within a state, country, or continent.

Jeffrey Howe, 41, a chaplain at the Salvation Army Red Shield Shelter on Broad Street, sees it as an opportunity to take kids on adventures to unfamiliar territory. The youths at the Broad and Fairmount shelter mostly come from the city, but geocaching has taken them to parks, the suburbs, and, once, to the 1,635-foot summit of the Pinnacle on Blue Mountain, in Berks County.

"Geocaching is a way to give kids the idea that there's a whole world out there other than what they know from their home neighborhoods and video games," he said. At the Pinnacle, "it was the first time most of them had seen deer, hawks, turkeys - we even saw a rattlesnake."

An avid geocacher on his own time, Howe pursues caches along hiking trails and shares the game with his wife and 4-year-old daughter. But he started geocaching with Red Shield youths when, during a weekly trip to a boxing gym, one of the boys noticed Howe's GPS in the van.

"There just happened to be a couple of geocaches on the way up to the gym, so we did those and they got so into it that we had to go out and look for several more," Howe said. "Those guys started telling other kids, and soon they started asking to go geocaching nearly every day of the week."

Geocaching has also caught the attention of educators such as Emilia Rastrick, a physical-education teacher at Lingelbach Elementary in Germantown. Rastrick, 37, applied for a grant for GPS units in an effort to combine environmental education with physical activity. She already has started teaching basic orienteering skills to her students, who are in third, fourth, and seventh grade, and plans to implement GPS work and geocaching in spring.

"The school is directly behind Wissahickon Park, so I'm constantly looking for ways to use the park to keep students' minds engaged while they're being active," she said. "There are several caches right around here, but I would really like to expand this into an actual outdoors club, where we could geocache outside of the city."

Geocaching within Philadelphia city limits has its appeal, too, particularly to history buffs. Old City is riddled with caches with names that suggest founding-father themes, while Rittenhouse Square and Fairmount Park also are popular destination stashes. Of course, the dense population in Center City makes stealth imperative, upping the ante for seekers attempting to find a cache without attracting the attention of "muggles," or nongeocachers. And most geocachers have had at least one run-in with police or security guards who find individuals sniffing around with GPS devices cause for concern.

How does it work?

If you log onto the Web site, you can enter your home coordinates or zip code, and then search for caches near your location. More than 18,000 caches are hidden within 100 miles of zip code 19130, for instance, all of which are named with given coordinates. Although posted coordinates will take a GPS within about 15 feet of a cache, a good hide will require seekers to do a bit of hunting around. Caches can't be buried underground, nor can they be hidden on private property, in dangerous locations, or in some national parks. A cleverly hidden cache, however, will cause its seekers to think creatively - and sometimes, to walk away disappointed.

The game attracts a wide range of personalities, from the self-described "techie geeks" to families with children to retired folks like Stephen Washburne, a former organic chemistry professor at Temple University. Washburne, 67, started geocaching in March.

"I guess I view it as kind of a disease," he said. "In less than a year, I've logged more than 450 finds, and looked for caches in 12 different states and three different countries. You get out there and see things you wouldn't otherwise see, and find things out about history and geography you wouldn't otherwise know existed. Sometimes people specialize in themes - all the caches in one particular area or state, for example. I also know one guy who only does cemetery caches - that's just his thing. I do it just to get out there and see places."

Posts written about the find - whether a searcher is successful or unsuccessful - are an integral part of the game, with users proudly cataloging the countries and continents to which they've traveled. Extreme geocaching, which involves strenuous terrain or special equipment, and puzzle-caching, which requires users to decipher a series of clues in order to find the final cache, also capitalize on the competitive nature of the hunt.

For many, though, geocaching is pure fun, or just a good way to meet other like-minded people. Ben Rhoades and Rachel Streck, both 32 and engaged to be married, met while geocaching in 2007. Rhoades, a volunteer geocache reviewer for eastern Pennsylvania, New Jersey, and Delaware, evaluates potential caches and hiding places proposed by individuals uploading coordinates on the Web site, deciding if they fit the guidelines to be added to the list of official caches.

"I've traveled widely while geocaching and I'm always interested in finding the interesting, offbeat place that the average local might know but the average tourist might never find," he said.

For Robert Broughton, 13, one of Howe's geocaching regulars, traveling around the city guided by a GPS is just part of an average week as a teenager. "I've already found all these, so I'm not supposed to help everyone else," he says quietly, while watching the others search for a cache in Fairmount Park. "I'm kind of an expert at this by now."