Now that the hot summer days are here, some people are looking for activities in which all the family can participate. A new outdoor game that is catching on here is called geocaching. It's a sophisticated high-tech scavenger hunt where the participants use handheld GPS (global positioning system) units to find the exact location of a hidden treasure. The electronic units allow you to determine your location on the planet based on co-ordinates given in longitude and latitude.

The word geocaching is a combination of geo for geography and caching for the process of hiding a cache.

The treasure is usually a small metal box filled with trinkets. When someone finds the treasure, he enters information in a logbook in the box.

One geocaching rule is that when a visitor takes something from the cache he should also leave something behind. People have left things like CDs, videos, games or even money for the next geocacher to find. So far, 3,613 active caches have been hidden across all 50 US states and in 56 countries, as far afield as the pyramids of Egypt and on an island in New York City's East River.

High-tech pursuits such as geocaching became a reality in May 2000 when the Clinton administration removed Select Availability, which was the intentional signal degradation introduced into GPS by the United States military.

Removal of Select Availability allows civilian users of GPS to pinpoint locations up to 10 times more accurately than they were previously able to do.

On May 3rd, 2000, a man in Oregon hid the first container to celebrate the removal of Select Availability. Mr Mike Teague found the container and built the first website to document these containers and their locations.

Last July, Mr Jeremy Irish, a Web developer in Seattle, Washington, discovered Mr Teague's website and his first cache outside Seattle. Recognizing the potential of the game, Mr Irish approached Mr Teague with a new site design called geocaching.

Individuals and organisations who set up caches just fill out an online form at geocaching.com where they enter the location coordinates of the cache.

Anyone can buy a GPS unit, which varies in price from $100 (#117) to $1,000, from outdoor stores or sites such as Amazon.com. A $100 GPS unit will allow you to pinpoint your location to within 20 feet.

To find the cache, a player enters the "waypoints" into his GPS unit. The unit can help him navigate from his current location to the exact location of the cache. Some complex units even have built-in topographic maps, electronic compasses and voice navigation.

In September, Mr Irish began to maintain www.geo caching.com, which is the official website for the game, and he introduced virtual logs and maps on the site. Although geocaching is still a part-time hobby for him, he told me in an e-mail that he spends 40 hours a week on it.

The game has caught on, he said, because people feel a sense of adventure without having to invest too much time and money.

"We all watch adventure movies and root for the good guy," he replied.

"We can't be Lara Croft from Tomb Raider or Indiana Jones, so this is a close approximation. It's a blast. All ages can play and it's relatively inexpensive. You also end up exploring areas that you may never have even known existed."

Geocaching rules are fairly simple. Anyone can decide where to place a cache and can also leave a note containing instructions about moving it to a new location. For example, a candle has travelled from Australia to the US and a Mr Potato Head has been doing the rounds from cache to cache.

Mr Irish estimates the average age of the players is about 29. "Some of the easier caches are being searched out by rookies, the less in-shape folks, families and couples with young children," he said.

"As they get more challenging you have folks who enjoy kayaking, hiking and off-road driving who play as part of their other activities."

Geocaching is not a moneymaking venture. Any money Mr Irish makes in shirt sales (and commission through Amazon's website) pays for the server maintenance and bandwidth. Additional money is put towards other items, like hats, that geocachers have requested.

"I don't make enough to live off," he said. "But it certainly helps to maintain the costs of running a website."

Mr Irish continues to hold down a full-time job outside of geocaching.

Since it's still a new sport, most people hear about the game through word of mouth. It is most popular in the United States but there are also pockets of players in Estonia, New Zealand, South Africa, Australia and in 51 other countries. A search of the Republic on geocaching.com showed three caches: one in Co Wicklow and two in south County Dublin.

Mr Chris O'Byrne, who placed two of these caches, said he watched the game grow in the US and, after a few weeks, saw that no-one in Europe had picked up on it. He hid Europe's first stash on Bray Head and his second stash (which has since become slightly defaced) on the N11 roadway near Loughlinstown.

He said he occasionally gets emails from people about the caches but "it doesn't really appeal to everyone - for starters, you have to invest in a GPS system before you can even consider joining in on the fun".

When Mr Kyle O'Connell bought his first GPS receiver he found information about geocaching on the Web. He has since hidden the Republic's third cache.

"I don't really think it has taken off yet in Ireland," he said. "Only three or four people have visited my site and to me this is a little disappointing. It is huge in the US at the moment."

When he inquired at shops like the Great Outdoors in Dublin, he was told they had sold "a good few" GPS receivers.

Even Hollywood is cashing in on the idea. Part of the marketing for the Planet of the Apes film due out on July 27th is to hide props from the film in certain locations around the world. Project APE is exclusive to geocaching.com and information about new caches will be released each week.

Down the road, Mr Irish expects geocaching to encourage the use of GPS units in geolocational entertainment.

"I believe this is the first game of many new ideas where you can use GPS technology and the real world as a game board. I expect the game will change and grow into new ideas that will be even better than this one."

Carol Power is a freelance journalist based in the New York area. She can be contacted at carolpower@ireland.com