In my hand is a clue that could lead me to buried treasure. It's written on a small card in heat-sensitive ink that will only reveal itself if I peer at it over a burning candle. As the words begin to emerge, I reflect on a bizarre day. Earlier, I decoded a cipher I found on a website, with the help of Kurt, a character who lives in a fictional place called Perplex City. I have also been out and about in London, looking for stickers on public telephones and lamp-posts that may betray further clues to the whereabouts of my prize.
Confused? Imagine how I feel. This is the world of "alternate reality games" (ARGs). Perplex City, the biggest game of its kind so far, has been puzzling 50,000 players across 92 countries. For the past 18 months, these people have been working together to solve thousands of riddles. In the game, reality collides with fiction via the internet, text messages, podcasts and e-mails. Thread all the leads together and the ultimate goal is to locate an object called the Receda Cube. The first person to find the burial site of this lump of precious metal - which could be anywhere in the world - will win £100,000. The final clue needed to locate this treasure is released this week, sending the hordes of dedicated players - and me - into a puzzle-solving frenzy.
The story goes that Perplex City is a fictional, futuristic metropolis, connected to, but not located on, Earth. The inhabitants take puzzles and mental pursuits very seriously, and their culture is awash with the cryptic and the mysterious. The Receda Cube, a scientific and spiritual artefact precious to Perplex City, has been stolen, and it's up to the game's players to locate it. The first sign of Perplex City in the "real" world was in 2005, when strange signs started appearing in phone boxes, on lamp-posts and in newspaper adverts saying "Lost - £100,000 reward" and listing the Perplex City website. Those with curious minds could log on to the site to learn more and start solving puzzles.
Perplex City isn't the first game to mix storytelling with treasure hunting. Michael Smith, its creator, is also co-founder of the online gadget shop Firebox.com. His other company, Puppet Master, wrote the rule book for ARGs. "The idea for Perplex City bubbled away for years in my head," says Smith. "When I was little, I was given Masquerade, a book by Kit Williams that was a kind of treasure hunt, with clues hidden in the pictures, I was fascinated by it, and wanted to create a modern-day version - that's Perplex City, 25 years on."
Thanks to broadband and a huge audience of tech-savvy consumers, Smith has been able to update the treasure hunt for the 21st century. "Instead of just using a book to disseminate the clues, there are so many different media channels we can use now. We've been hiding information in websites, in text messages, in podcasts, in online videos, we've put clues in newspapers and magazines, and created a massive cross-media adventure game."
Of course, Smith isn't the first person to create a multimedia mystery game, but his is certainly the longest-running. Other popular ARGs have been used as "viral" marketing tools, a way for companies such as Microsoft and Nokia, and the hit TV show Lost, to get inside the minds of punters. Such games tend to run for two months, but Perplex City isn't a promotional gimmick, it's big business. Many clues come on cards that cost £2.50 a time. So far, Smith has notched up £1m in sales. No wonder he following up Perplex City with more of the same.
Like Second Life, the popular website that transports people into a virtual world, Perplex City gives players the chance to interact within an altered reality. Smith is aware of the similarities. "Second Life is a great way for people to connect, but it's restricted to an online world. Perplex City is played online but it spills into the real world. That's what makes it more interesting to a larger group of people - the online and offline crossover."
Perplex City also has its own online newspaper, the Sentinel, for players to keep abreast of what's going on in the city. It's this level of detail that appeals to players. Chris Warren, 27, from Edinburgh, has been playing Perplex City for six months. "As well as the puzzles, which I love, there's the story, which is well written and draws you into that world, like a soap opera. The players are a surprising mix, a huge range of ages and professions."
Warren is tackling "Billion to One", a challenge on puzzle card No 256. "We've been asked to find a man, and all we've got is a photograph of him. He could be anyone, anywhere. I'm working with other players to find him, spreading his photo around the globe."
No wonder people need an online community to help them out - at first glance, the puzzles seem incomprehensible. Once you get some expert advice, though, things slowly make sense. Warren explains how the game works. "There are two sides - the puzzle cards, which you can buy and win points by solving, and the mystery of where the cube has been hidden." Solve one clue and you find the next, until, finally, you may arrive at the treasure. In the meantime, you accumulate points, climb the leaderboard in the Sentinel, and earn access to new clues.
The puzzle cards act as an introduction to the characters and story of Perplex City and the mystery of the stolen cube. The cards have difficulty ratings - red is the simplest, orange and yellow are trickier, green is hard, blue and violet are almost impossible, and the less said about black and silver the better. The tougher, rarer cards change hands for real cash - one sold for £521 in a charity auction. Each card has a scratch-off coating hiding a number that is needed to answer online and claim the points.
It takes lateral thinking, the ability to crack optical illusions, word-games and algebra to solve the brainteasers. But it's not all sitting at home and scratching your head. Events have included a competition to build the highest tower out of straws, a "dance off" in London's Trafalgar Square and a scavenger hunt across downtown New York. Winning these real-world challenges is another way to climb the leaderboard and get an early preview of the next clues.
But what happens when the cube is found? Smith won't be taking a break from tormenting players with his devilish riddles. "We didn't want this to be just a flash in the pan, so we tried to build a world and create characters that would live on in new stories. "We refer to 'The Search for the Cube' as 'Season One'," he says, "and we're launching 'Season Two' in a couple of months. It will be a whole series of new mysteries." So, keep your eyes peeled for strange messages, and you could be in at the start of the next hi-tech hunt, and even end up £100,000 richer.
What is Perplex City?
A treasure hunt for the technological generation, run by a company called Mind Candy. Players solve clues, found in real locations (stuck in phone boxes or on lamp-posts) or on websites, in text messages or on collectable puzzle cards that can be bought from www.firebox.com for £2.50. They then go online to collaborate with other players. The aim of the game is to find a mysterious cube stuffed with £100,000. If they're successful, how they share the cash between collaborators is up to them.
Other alternate reality games
Launched earlier this month by Microsoft to promote its new PC operating system, Windows Vista. As might be expected, the game is being played on a grand scale and one of the prizes is a trip into space. Visit www.vanishingpointgame.com to get involved.
Perplex City Season Two
For another chance at a solving fiendish problems and winning a big cash prize, would-be treasure hunters can try their luck in the next version of Perplex City which will start later this year. www.perlexcity.com
Evidence: The Last Ritual
Load the CD on to your PC and then go online to start solving a series of fictional murders through puzzles that are delivered to you through e-mail, videos and music files. A serial killer is on the loose and players have to gather evidence to find the perpetrator and stop him. It costs $29.99 (£15), from www.adventurecompanygames.com
For teenagers, Cathy's Book is a hands-on mystery. Players receive a notebook of drawings and notes plus a packet of "evidence", from scrawled notes to discarded napkins. Exploring the web and making calls, it's up to you to get to the bottom of the disappearance of Cathy's boyfriend and other strange goings on. $17.95 (£9), from www.cathysbook.com