There are loads of things that make my dad shout, but the internet usually works best. That and instruction manuals. This weekend, we discovered something that actually involves both: it’s called “geocaching”. Mum says she’s going to get us all earplugs if we ever try it again.

I heard about geocaching from my friend at school. It’s just like a giant treasure hunt, with more than half a million “geocaches” – basically Tupperware boxes containing little bits of treasure – hidden around the world. Apparently, there are about 25,000 in Britain, all hidden by geocaching fans: at waterfalls, on top of mountains, in city parks, anywhere, really. And only geocachers know they’re there. It’s like a secret club.

My friend said you go to www.geocaching.com and up pops a map with all the geocaches near you. Then you get a GPS, enter the coordinates of the geocache you want to find, and off you go. “Sounds awesome,” says my dad. “Sounds ominous,” says my mum, which is what she always says when dad gets excited about computer-type stuff, like cameras and iPods.

“I can’t find the damn map!” Dad shouts from the study after only about 15 minutes on the geocaching website. “It keeps taking me to Alabama.” Actually, the website is quite annoying. If you type in your postcode, it says it can’t find you. If you click the UK link, it comes up with millions of caches, none of them anywhere near your home. Eventually, Dad tries putting a gap between the ‘FK8’ and ‘3JZ’ of our postcode. A list pops up, with a link near the top of the page saying: “Search for caches with Google Maps.” Dad clicks the link, and up pops a map of our area. “Oh my God,” he shouts, “there’s billions.”

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Next, we borrow a GPS and tap in the coordinates of a geocache close to home. “What?!” Dad shouts. For “distance to destination”, the GPS is telling us 213.7 miles. “Are you sure you put the coordinates in right?” asks Mum. “What do you take me for?” Dad replies. Mum just raises her eyebrows.

Eventually, Dad works it out. Turns out they list two sets of coordinates on the geocache website. He’s been entering the American kind when he should have been entering the British ones. “It’s bad web design, really,” he says. Mum looks like she’s about to say something, then just looks at me and smiles.

As we drive towards the geocache, I get to hold the GPS. It’s amazing, like playing warmer, warmer, with the GPS counting down the miles. There’s even an arrow telling us which way to go. Also, we have a coded clue from the website. “Oruvaq ebpx haqre fznyy zbffl fghzc,” it says. Mum says it probably means “Smile, you’re on Candid Camera”, but, once we’ve swapped the first 13 letters of the alphabet with the last 13, it turns out to mean “Behind rock under small mossy stump”. “That’s helpful,” Mum says. “There aren’t many rocks or mossy stumps in Scotland.”

We park exactly 0.21 miles from the geocache and stand around looking at the arrow. “It’s pointing at the river,” I say, which is weird, because there’s no footpath going along the river, just lots of pine trees with spiky branches pointing at your head. Dad takes the GPS off me. “Just until we know where we are,” he says. In the end, after Dad’s looked at the map and told Mum she’s definitely wrong about the path we passed in the car on our way up, we try the path we passed in the car on our way up. “It’s switched to feet,” Dad shouts. “We’re closing in.”

Ten minutes later, we reach a waterfall, which is where the website said we need to look. Dad’s given me back the GPS, and it says we’re within 16ft. I follow the arrow some more – suddenly, it says 25ft, pointing back the other way. It’s gone mad, spinning about all over the place. Every time I walk one way, it tells me to go back the other. Dad says the trees are blocking out the satellites: the GPS doesn’t know where it is any more. “Let’s just look for a rock under a small mossy stump,” Mum says. Dad shakes his head.

It’s really exciting at first, looking under every stump, but there are just so many of them, then the baby starts to cry and my brother, Joe, keeps wandering too close to the river. “Never mind,” Mum says. “It’s a lovely spot, and we’d never have come otherwise.”

Suddenly, Dad starts jumping up and down: “I’ve found it!” he shouts. We look around to check that nobody’s watching – the website tells you not to let geomuggles catch you at it – then we all go over to open up the box.

Inside are all sorts of kids’ toys. We swap a mini water pistol (for Joe) and pencils (for me) for some snap cards and slime. I get to hide the box back in its hole, and suddenly it all feels totally amazing: people could sit on this stump and never even know. It’s how Harry Potter must have felt on Platform 9¾.

Back home, we go straight onto the geocaching website to log our find. I want to find out how you can bury your own treasure. Dad wants to find out the difference between a multicache and a mystery cache. Mum wants to see if there are any geocaches in France that we can visit when we’re there at half-term. Then we get an e-mail. It’s from the man who hid our geocache by the waterfall! “Welcome to the madhouse,” he writes. “Be careful, geocaching can become addictive.”

“What does addictive mean?” asks my brother, Joe. “It means we’d better buy our own GPS,” says Dad. “Sounds ominous,” says Mum.

Which GPS?

The short answer is that any GPS will do. Even the most basic model will count down the distance to a geocache and point to where you have to go. As in life, however, the more you spend, the more fun you’ll have.

Less than £100: the best budget option is Garmin’s eTrex H. It’s waterproof, it fits into the palm of your hand and it uses a WAAS-enabled receiver, which means it’s accurate to within 10ft about 95% of the time – even in gorges and forests. It doesn’t have a base map, so it won’t show what lies between you and your geocache, but use it alongside an Ordnance Survey map and it does the job. £75, from GPS Warehouse (www.gpsw.co.uk)

£100-£200: the next step up is something with a base map, showing roads, rivers, street names and so on. A good option is the Magellan eXplorist 210, which has 22MB of internal memory for downloading topographical (“topo”) maps from the internet.£150, from Sporttek (www.sporttek.co.uk)

£200-£300: a device in this bracket has everything we’ve mentioned before, plus colour screens and built-in slots for SD memory cards, so can you load an OS map covering the whole of Scotland directly onto the GPS. Try the Road Angel Adventurer 7000.£250, from Maps Warehouse (www.maps-warehouse.co.uk)

£400: just out, the Garmin Colorado 300 is a geocacher’s dream, with all the above and more. It lets you download descriptions and coordinates direct from the geocaching website, so there’s no scribbling them onto paper. Acting like satnav in your car, it is also the easiest GPS to use one-handed in the field, because of its clever control wheel. £400, from Cotswold Outdoor (0844 557 7755, www.cotswoldoutdoor.com)

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