Geo-caching has roots in the 150-year old English hobby of letterboxing, in which clues are given mostly word-of-mouth and participants leave their mark with rubber stamps.

Although letterboxing still has enthusiasts, modern-day geo-caching began in 2000, when the government made accurate GPS more accessible to non-military personnel. To celebrate, computer scientist Dave Ulmer hid trinkets and a logbook in woods near Portland, Oregon, and then posted the location to an Internet newsgroup frequented by GPS users.

Since then, geo-caching has proliferated worldwide, with new technologies spawning increased interest -- enthusiasts now can even geo-cache from I-Phones.

Although geo-cachers on the whole are an earth-friendly group, the activity has drawn criticism from folks who see caches as a type of litter. Some lands, including national parks, are off-limits to physical caches. Instead, geo-cachers are invited to solve puzzles aimed at tree identification and other educational activities.

"People obviously have debates about the litter issue," said Jen Sonstelie, a spokesman for Groundspeak, which owns www.geocaching.com. "We encourage 'cache in, trash out,' which means picking up trash during your geo-cache adventure."

Other concerns are that geo-caching encourages trespassing and earth disturbances.

"Geo-cachers are supposed to get permission from property owners or confirm public access is allowed," said Sonstelie. "They're also not supposed to bury their caches underground. They can cover a cache with leaves or put it in a hollow tree, but no digging is allowed."

They also should avoid hiding cache near schools, as well as places considered unsafe, such as under bridges, she said.

Cache owners are expected to deposit only family-friendly items, and no food or food-scented items that could attract wildlife. They also must maintain their cache, which means checking on it periodically and removing filled-up logbooks, Sonstelie said.

Although a team at www.geocaching.com reviews cache requests, ultimately geo-cachers operate on the honor system, and the global community is a self-policing group, Sonstelie said.

"Considering how many people geo-cache, we've had very few strange incidents."

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