Roughly 374,000 people ride BART every day, which means that a single ad on a moving train can catch hundreds of eyeballs. Such was the thinking of Truther organization Rethink911, whose members wanted to maximize their audience by purchasing 100 ads on the Bay Area transit system. Decked with before-and-after photos of the original World Trade Center Building 7 — which burned down seven hours after the Twin Towers, in what some people believe was a controlled demolition — the ads ask, mysteriously, "Did you know a 3rd tower fell on 9/11?"

Although investigators from the National Institute of Standards and Technology deemed that idea specious, members of Rethink911 — whose ranks include architects and engineers — believe that, with the right advertising campaign, they might be able to sway intellectually curious populations in cities like San Francisco. And evidently they think the most curious among us take public transit. Thus far, they've plastered buses in Ottawa, bus shelters in Vancouver, and street boards in London. They also installed large billboards overlooking Times Square in New York and the Stemmons Freeway in Dallas. That's enough ads to create thousands of impressions per minute in some of the world's most populous areas.

"We knew we wanted to do a lot of cities," says Rethink spokesman Ted Walter, who watches people's reactions to the ads while taking BART to his home in North Oakland. "We wanted to cast a wide net — because who knows where the local media might be interested?"

Well, touché. Even if the orange and blue posters aren't particularly arresting, they still constitute a valiant form of evangelism. Building 7 theories are a hard line to sell more than 12 years after the event, and Rethink was stymied in certain markets where the transit systems, or their ad contractors, ban political ads altogether. In Boston, Chicago, and D.C., they shunted the campaign over to taxi cabs.

San Francisco Supervisor Scott Wiener tried to enforce similar prohibitions on Muni after a May dust-up over Israel/Palestine propaganda. But that proposal failed, and as a result, San Francisco has been one of the Rethink campaign's most fertile markets, although members ultimately picked BART over local bus lines. (Both systems contract with the same company, Titan Outdoor, which tolerates most forms of free speech.) At $12,500 — all donated by supporters — for 100 trains, Walter says the BART package offered a better value proposition.

"We felt we'd get more bang for our buck," he says. "We felt that if we raised awareness, then a majority of people would suspect that Building 7 didn't collapse due to fire, which would lead to an outcry for another investigation. And then the house of cards would begin to fall."

And that, we assume, is the power of advertising.