If these passages are "secret" then how would Gullible.info know about them?

Great question Neil. You're absolutely right to wonder how we compiled this number. One of the way we research things here is by working backwards. We'll figure out what we want to say, and then pour through data to try to piece together the information. This fact is an example of a case where we used that particular methodology.

I have long been fascinated with secret passages. So much so, that the house I commissioned in my mid-20s had 800 square feet of "secret" space, including a 500 square foot study. I am only telling you this because I have since sold that house, and because it illustrates my fascination with hidden rooms and walkways.

Since we began our fact proffering service a little more than a year ago, I have been trying to calculate the square footage of all secret passages in the US. Obviously this is a difficult task. For one, the passages are secret, so we can't just go around and ask. However, luckily for us, I have close professional ties to the head of the North American Association of Demolition Technology. He and I did a little study over the last six months, totaling all hidden square feet in the buildings his members demolished.

Charting these data out based on the year the house was built, we saw a very clear trend: older houses have more passages than newer houses. This wasn't a shock to either of us. What was a shock, though, was the tightness of the trend (correlation: .905). Extrapolating our numbers out -- the newest house included in our data set was built in 1995, and had 0 square feet of passages -- we applied the trend to the number of buildings currently standing in the U.S., accounting for their construction year, we came to the total of 29,831,221.64. Rounding up, we got 30 million.

I hope this answers your question. Taed is right, too. We got the total number of buildings as well as their age distribution from a FOIA request.