I am. I am. I am 28. I am 34. I am from Portland. I am from Tokyo. I'm happy today. I am from the US and I am naked. I am bi.

On a grey San Francisco day last October I sat alone in a darkened room at the city's Yerba Buena Centre for the Arts, watching streams of illuminated words and phrases passing before me on a bank of small screens. As they flowed before my eyes, they were intoned in a HAL-like computer-generated voice (with a curious touch of Cary Grant), accompanied by a haunting piano score. Half an hour later I hadn't moved. I was transfixed.

This is Listening Post, a piece of techno-art by the sound artist Ben Rubin and the statistician Mark Hansen, both 43, that has been nearly a decade in the making and which, after numerous updates and a stint at the Whitney Museum in New York and the YBCA, has now found a home as part of the permanent collection at the Science Museum in London.

The key to the piece is that the messages you see and hear flowing across the grid of 231 vacuum tube screens (the type of screen you see on cash registers) are derived from a continuous live feed from thousands of internet chatrooms. As you watch, you slowly become aware that what you are seeing and hearing is, effectively, a snapshot of the world talking. It is strangely moving.

It's also ground-breaking as the first work of art to communicate effectively the scope and nebulous nature of the internet. In a review of the art/technology festival Ars Electronica in 2004, where the piece won the Golden Nica award for interactive art, Michelle Kasprzak of Mute magazine described it as a “monument to the present”.

A computer attached to the screen-bank takes all the messages posted in the preceding half hour from several thousand English-speaking chatrooms across the internet, then sorts this raw data according to one of seven programmes that Hansen has developed, before displaying the results on the screens. Each of these programmes is behind a different “movement” of the piece, and each has a different intention: one, for example, picks out only screen names for display - toothlessblond, whoever you are, you made my day. The feed of data is continuous, so it never repeats itself.

“The icons of the 20th century, such as Stephenson's Rocket and Apollo 10 are big, beautifully crafted objects,” says Hannah Redler, the Science Museum's head of arts projects, “but contemporary science and technology is very much in the micro-world, the world of concept. It's a constant challenge for us to find things that communicate science in themselves, and this does that.”

This isn't how Listening Post started, however. When Rubin met Hansen at a conference designed to introduce artists to researchers at the famous Bell Labs (out of which have come the transistor, the laser, information theory and the Unix operating system, to name but a few innovations), their humble intention was to develop a way of displaying data using sound, instead of the more usual visuals - graphs, charts, 3-D fly-throughs and so forth.

“We started with a series of experiments of looking for data that we could listen to and trying to write software and see what we could make it sound like,” Rubin explains. “We started by sonifying data that was available to Mark, which was logs from the Bell Labs website. We found that the sounds were nice but, really, who cares? Who wants to listen to that?” Rather than something abstract and numerical, Hansen suggested that they try chat, using words as a data source.

The physical aspect of the piece - the bank of hanging screens - evolved from Listening Post's first demonstration at the Kitchen, an experimental performance space in New York. The pair intended just to play the sounds they were working with, hooked up to a live chat source but, while they were testing it, they were struck by the text flowing up on their computer screens and realised that the power of the piece was amplified when a visual component was introduced.

Visiting Listening Post is a disconcerting experience. It is housed in a darkish room, bathed in the cool green glow of the text flowing across or down the screens.

There are seven movements, which run in succession. The most complex movement scans incoming messages, pulls one at random, then scans the subsequent incoming messages to find one that has similarities - words or groups of words. If it finds one, it starts working on an adjacent screen to find another message that follows on from that one.

“Some, which are kind of oddball messages, just die because there's nothing really like them, but if someone today is talking about the US presidential nomination results, there are going to be others latching on like crazy and you get these clusters of messages that relate to something going on that day,” Rubin says. Hansen found out about the US space shuttle Columbia exploding while doing maintenance on Listening Post at the Whitney, just by running this programme.

Another movement, and probably the most affecting, is “I am”. After doing an analysis of the initial words of chat messages one day, Hansen found that “I am” (or “I'm”) was the most common beginning of a posting by a wide margin. He took all the “I am” posts from that day, sorted them by length and e-mailed them - all 60 or so pages - to Rubin. “It was the most incredible poem,” Rubin says.

The “I am” movement does the same, starting with the shortest messages and gradually increasing in length. Listening Post's calm voice (a standard computer-generated one, it pronounces FCUK as “fkuk”, rather sweetly) reads them as they appear, and they silently repeat on their screens as more are added. They range from the innocuous, through the racy, the poignant - “I'm used to guys being nasty to me” - and the funny, to the downright bizarre. It feels voyeuristic.

And this is where the impact of Listening Post lies. Watching and hearing all these confessional snippets, you can't help but extrapolate stories, feel sad for that poor unlucky girl (or guy), snigger at the naked American sitting starkers at his computer. Thousands of human stories appear, fragmented and fleeting, and then they are gone. It's the closest you can get to eavesdropping on everyone, everywhere, all at once. And that is pretty overwhelming.

Listening Post is at the Science Museum, London SW7 (0870 8704868)

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