Ten years ago, an American Army computer engineer hid some items in a bucket and plotted their location using a Global Positioning System, for others to find.
Now, geocaching is a worldwide phenomenon.
Ian Peacock investigates.
There are now over a million caches in the world, some that travel, some stationary - there's even one at the bottom of the Antarctic.
It's a multi billion dollar industry that has got billions hooked.
It's also pervasive - the tourism industry and educationalists are embracing it.
Ian begins his quest as a 'muggler' - a non-geocacher.
So it's with some scepticism that he begins his first hunt for a cache.
After all, he wonders, where's the challenge in finding something that you have been given the exact co-ordinates for? His first search takes him to a dingy bridge in Bristol.
The cache reveals a slightly disappointing horde - a 10% discount sandwich voucher.
But his geocaching mentors, whose handle is 'The Bloringers', try to convince him that's it's all about the journey, rather than the prize, and it's not long before Ian begins to get hooked.
He and his producer decide to have a race.
A travel bug race; they set two moveable travel bugs in separate caches to see whose bug travels furthest and fastest.
Some travel bugs make round the world trips.
One couple even let the final destination of their travel bug determine their honeymoon destination.
Ian also sets our very own Radio 4 audio-cache.
People who find the cache will leave a short recording about their joyful experience of hunting for it, or so he hopes.
For some, geocaching really is their life.
Ian tracks down the internet millionaire who pursues geocaching as his full time hobby, and who holds the British record for number of caches found.
An autistic student explains why geocaching is so appealing to him.
Geocaching is a very sensitive subject for some.
The furtive nature of urban geocaching (necessary to avoid vandalism to caches) can arouse suspicion, and people adopt disguises to go unnoticed.
But it seems that geocaching is an unstoppable force.
Ian talks to the people who make money from it, the people who embrace it as an educational tool, but mainly to those who just love doing it!
Producer: Sarah Langan.