A location system using orbiting satellites originally developed by the United States military is now the main tool in a grown-up game of hide-and-seek.

Geocaching (geo-cashing) uses hand-held global positioning system (GPS) receivers to find hidden "treasure" hiding in trees, under rocks and along city streets throughout the world. About 4,023 such stashes exist within 100 miles of Prescott ­- including at least one site in downtown Prescott - with "cachers" adding more all the time.

GPS enthusiast Dave Ulmer created the game within days after the United States government flipped the switch on its worldwide global positioning system to let U.S. civilians with GPS receivers - and the rest of the world - get accurate readings. That was on May 2, 2000.

Until then, the U.S. military scrambled the signals to avoid putting pinpoint accuracy into the hands of possible enemies.

Today, thousands of people worldwide are geocachers, including Prescott residents Bob and Linda Smith.

All participants need is a hand-held GPS system and a computer. By logging onto www.geocaching.com - or any other geocaching site ­- and becoming a member, people can enter a ZIP code and find the longitude and latitude of any site within 100 miles.

Organizers prefer not to publicize the exact locations of the sites in fear that someone might "muggle" them, Bob Smith said.

"We try to be careful when we're going after caches," Bob continued. "We don't want to expose them to 'muggles'."

The geocaching community has adopted the Harry Potter term as a description of people who either do not understand or do not respect the sport of geocaching and destroy the sites.

Each site has a rating of the difficulty of finding it and according to the difficulty of the terrain.

One site on a rock in the middle of Watson Lake rates as 4.5 out of 5.

"I haven't gone for it - yet," Bob said with a gleam in his eye.

A 5 rating usually means the site takes special equipment - such as climbing or scuba gear - to find. Bob says some people have even used hot air balloons to find caches. The most elaborate equipment he and Linda have used is a kayak.

Some sites have a name and some names are clues to help find the site, Bob explained. Sometimes the write-up about the site contains a hint and sometimes the hints are in code.

"That's what's so much fun is reading some of the logs that people place on the website," Linda Smith said.

Every geocache "belongs" to someone who has the responsibility to maintain it, Bob said. Each person who takes something from a site is supposed to leave something in return.

Bob owns several cache sites himself. The most important item in each cache is the log sheet. Each person who finds a site initials and dates the log inside the cache container. When they get home, they enter the finding on whichever geocaching website they belong to.

Some people strive to find as many sites as they can within 24 hours. That is called power caching, Bob said. The most he and Linda ever have found in one 24-hour period is 17.

Anyone with more than 300 total "finds" to their name qualifies as a real cacher, Bob said. Together he and Linda have found 375.

In the early days the thrill of geocaching was in finding and trading the items found at the cache sites. They might be trinkets or even larger things, depending on the size of the cache container. Some containers - called micro caches - are very small. Nowadays the important thing is to find the cache, log it, and log the find, Bob said.

Linda - who enjoys hiking - explains that geocaching is a healthy, family sport that anyone can enjoy.

"There's places I've seen that I wouldn't have even known about except for geocaching," Linda said.

The most exotic place they have ever geocached was in Alaska. They were on vacation and got off the plane there at 5 a.m. Since they did not have much to do for a few hours, they whipped out their GPS receivers, plugged in the coordinates they had brought with them, and off they went into the wilderness.

Intent on following the path they were on, they did not look up until it was almost too late.

"There was a huge moose looking at us on the pathway right where the cache was," Linda said. It had a calf with it.

"We didn't find that one," Bob said laughing.

Contact the reporter at tshultz@prescottaz.com