People are playing near you right now.

Wherever you are right now, chances are you’re steps away from a mystery that people around the world are trying to solve. I’m talking about global scavenger hunts, which were created by the internet community a couple decades ago and have grown in popularity since then. The hunts have inspired people to get up from their desks, phones in hand, and go searching for hidden treasure.

It’s called Geocaching.

Back in the spring of 2000, at the direction of then-president Bill Clinton, the US government downgraded the security of its 24-satellite global-positioning system?—?which it had used to track military vehicles, among other things?—?so companies and regular people alike could use it. People flocked to the technology. One of the ways they used it was to create a game.

‘Cache’ is pronounced ‘cash.’ | tyler olson / Getty

In May of the year 2000, computer consultant Dave Ullmer chose to test the accuracy of the government’s GPS by hiding a black bucket in a forest near Beavercreek, Oregon. Ullmer posted the bucket’s GPS coordinates online in what he called the Great American GPS Stash Hunt. The bucket contained a notebook, a slingshot, books, videos and software. Two people found it, and in the week that followed, other internet users began hiding their own caches and posting their coordinates to a mailing list. One user coined the term “geocaching” to name the budding pursuit.

A geocache and an ancient ‘GPS device.’ | Richard_Pinder/Getty

The list grew, and in September 2000, the Seattle-based web developer Jeremy Irish built a webpage called geocaching.com, which became the home for the hobby. Today there are almost 3 million geocaches around the world. One is almost certainly near you. All you have to do is register on the site, search in your area and see what pops up. Grab a bottle of water and a smartphone and see if you can find the cache.

This short video video explains it all.

Some geocachers create avatars, as these Scouts did. | QT.Coon

What are you looking for?

The simplest and most common geocache is a container that holds a logbook?—?which you sign when you discover the cache?—?plus one or more additional items. Take one, and leave something of equal (or greater) value for the next person. While hiders can be creative?—?crafting containers from film canisters, hats or even a half-buried Volkswagen Beetle?—?one of the most popular vessels is a simple army surplus ammunition box:

It used to hold bullets. Now it holds mystery. | ra?—?photos/Getty

“Nanocaches” are much smaller. They’ve been hidden inside tennis balls, Altoids tins and even fake birds’ nests. They usually contain just a small note on a slip of paper. Photograph yourself with the nanocache to log your accomplishment.

Hard to find, easy to swallow. | Jack_Art/Getty

Use your brain

“Puzzle caches” require you to use extra brain power. Sometimes it’s a device you must decode to unlock. Other times it’s a logic puzzle. You might even have to wire a circuit.

Nobody said there’d be math involved! | DaanPhoto/Getty

Some caches don’t feature containers at all. An “earth cache” is just a geographic location. To claim the cache, you answer questions about the place that can only be answered by someone who’s visited it, or photograph yourself there. An earth cache might be a simple rock formation or waterfall, or a monument in Ukraine.

“I had to go to a small park and find a rock and locate a little tiny fossil on it. Instructions said I had to take a photo and send it in. It was very cool because it taught me about fossils and geology. So not only are you geocaching as a game, you’re learning about history or you’re learning about our environment.”?—?Stephen Watkins, aka QT.Coon, geocacher

Found the earth cache! Now how to get there? | mel?—?nik/Getty

How hard is it?

Caches are rated by difficulty. A level 1 cache might simply involve walking into the woods and locating a birdhouse. More challenging caches have been hidden in treasure chests at the bottom of the sea (accessible only with SCUBA equipment). Dozens of geocachers have found a cache in the Registan Desert in Afghanistan, but one hidden at the Bagram Air Force Base was confiscated by military police. The very trickiest include one in Arizona that requires cachers to jump a four-foot chasm?—?that’s 100 feet deep?—?and another at the summit of Mount Everest.

The prize for most remote cache has to go to Ultima video game designer and geocaching pioneer Richard Garriott, who journeyed to the International Space Station as a space tourist in 2008. While there, he hid a cache in a locker on a Russian section of the station. Better get to work on that astrophysics degree!