Josh Harris is “the greatest Internet pioneer you’ve never heard of.” That’s true, despite the sheer amount of press he received in all his ventures, both online and off. That says a lot about how fleeting fame can be when tied to the Web, and the 2009 documentary “We Live in Public” — showing Wednesday at City Arts Center — explores the degrees and dangers of that visibility.

Harris’ rise and fall was so quick, that if charted, they might look like an A. As director Ondi Timoner (“Dig!”) notes, he was among the first to provide hard numbers on Internet statistics and trends early on, making him his initial fortune. He made his second by founding Pseudo.com, a trailblazer in ’net programming.

But what he did after that gives “Public” its power. Believing the world was moving toward a time where people would give up privacy online, he commissioned an “art project” in which 100 volunteers were sequestered underneath New York City. They wore matching uniforms, lived in pods, and were subject to intense interrogation and under the watchful eye of cameras for everything they did. Yes, everything.

With food, drink and drugs provided gratis, the experiment began as quite the party. Before the month was up, however, it had turned into “Lord of the Flies.” Some of Timoner’s footage is deeply disturbing, as the humans began turning into animals, and everyone is too numbed to notice or do anything about it. (Hey, who thought including a gun range was a good idea?)

After the cops shut that down, Harris went on to do the same thing on a much smaller scale: just him and his girlfriend, Tanya, in their apartment, with dozens of motion-sensor cameras capturing their every move. Yes, every — there was even one installed inside the toilet. Again, it’s all fun until someone gets hurt, and that person is Tanya, when he suffers a breakdown and lashes out at her physically.

“Public” is one of those documentaries that succeeds because of Timoner’s unfettered access to her subject, over the course of a decade. Harris makes for a remarkable focus — part prophet, part madman, as troubled as he is intelligent. The film is fascinating, although too upsetting to be entertaining. You won’t want to avert your eyes, but you’ll never want to see it again. And some of it sticks with you so strongly, you may never need to. —Rod Lott