Hide-and-seek for nerds

Geocachers search for buried 'treasure' using global positioning. Clint Witchalls gets digging

You may well ask yourself, what the hell is geocaching? Well, it's the new sport for the cybergeek. It goes like this. You hide some "treasure" - any old junk - in a public place. Take the co-ordinates of the place using a hand-held global-positioning system (GPS) device. Put the co-ordinates on to a website, for example www.geocaching.com, plus a cryptic clue, and let other people search for it. It's a hi-tech treasure hunt, except without any real treasure.

I borrowed a Garmin GPSmap 60C hand-held device for my adventure. These gizmos, which look like obese mobile phones, were designed for mountaineers, sailors and outdoorsy people, but, like all new technology, people find unexpected uses for them. Criminals are usually the first to find novel uses for gadgets, but in this case the nerds got there first.

GPS was initially developed by the American military in the late 1970s. Today, it consists of a constellation of 27 satellites (plus a few back-ups) that transmit high-frequency, low-powered radio signals. A GPS receiver scans for these signals and uses a mathematical system called "trilateration" to work out your location. As the exact position of each satellite is known, as is the speed at which radio waves travel, the GPS receiver only has to work out how long it took for each of the three signals to reach you in order to calculate your exact longitude and latitude. Well, not "exact", but they are accurate to within about three metres. With a fourth satellite signal, it is also possible for the GPS device to work out your elevation - useful if you're a mountaineer or skier.

Initially, hand-held GPS devices were expensive and, indeed, the model I borrowed retails for £455. But there are many low-cost devices, such as the Magellan eXplorist and the Garmin eTrex Personal Navigator that can be bought for under £100. The development of these low-cost hand-held devices has done a lot to fuel the interest in geocaching. A few mobile phones are also GPS enabled, and some video-game makers are working on hand-held GPS games. But none of these gizmos would be around today if it weren't for the Clinton administration deciding to unscramble the radio signal sent from military satellites, making GPS technology available to the hoi polloi.

Up until 2000, the US military satellite network worked on what was called "selective availability". Selective availability meant that the military could use GPS with an accuracy of 10 metres, but civilians could only get a location accurate to 100 metres - not much use if you're looking for a small plastic box in a church garden. Shortly after the high-frequency radio signal was unscrambled, David Ulmer of Portland, Oregon, hid the first geocache and posted the co-ordinates on a newsgroup. He challenged anyone to find the stash, which consisted of a can of beans, a slingshot, $5, mapping software and a log book.

The very next day, Mike Teague, a Washington resident, found the hidden box - and the sport of geocaching was born. There is now a plaque to mark the spot where Ulmer's cache was found. The sport has grown hugely since then. At the time of writing there were 193,540 caches in 217 countries, including 29 in Iraq. In the UK, there are more than 7,000.

If you want to play the game, you'll need an internet connection, a hand-held GPS device, and a childlike sense of adventure. The first thing to do is to decide where you want to hunt for your cache. You go to www.geocaching.com/seek/ and enter your postcode and the radius from that location that you're willing to search for Tupperware boxes. You click the search button, and a list of locations appear. These locations, called "waypoints" or "geocache points", can be downloaded to your GPS receiver via a USB cable, or simply typed into the device. The person who hid the cache may also provide a cryptic clue. For example, a clue for a waypoint in Iraq is: "You have to use this to clear your weapon. Located on east end of parking lot." I chose a slightly less dangerous location in the City of London. It was quite easy navigating from Bank Tube station to the park where the stash was hidden, even though the satellite signal was weak, due to the tall buildings. Although I ended up soaked and muddied, it was an interesting trip. I've lived in London for 15 years and I'd never heard of Postman's Park, the tranquil oasis where the stash was hidden. I never did find the Tupperware box, even though I cheated and tried to find the gardener, who apparently knows where the cache is hidden.

I might have been more tenacious had the Tupperware box contained £1,000, but I knew that previous players had left things such as pens and one Swedish krona, so I decided to call it a day. I justified my actions by convincing myself that the cache had been "muggled". Muggled is a term used by geo-nerds to signify a cache that's been removed by ordinary folk - although who'd want to nick a box full of tat is a mystery to me.

Geocachers would undoubtedly console me by saying that the game is about the journey, not the destination. And in that sense it was successful. I saw a part of London that was entirely new to me, but I could easily have done this without £455 worth of equipment. All I needed was a guidebook and a travel card. Like other outdoor pursuits, geocaching appeals to a limited number of people. Unfortunately, I'm not one of them.