The following year, Diamond started the Brooklyn Historic Railway Association, a nonprofit company dedicated to preserving the tunnel’s history. He also got the tunnel added to the National Register of Historic Places. And he started hosting tours. Now, almost 30 years later, the popularity of the tunnel is growing. Tours that were once given sporadically (around three times a year) now take place twice a month — with two scheduled tours a day. Each tour is capped at 100 people, and the tours are always full. Folks line up in front of the old Independence Savings Bank building that was recently turned into a Trader Joe’s supermarket. The line snakes past what was once the bank parking lot, where luxury condos now stand. Change and the transformation of historic buildings is ubiquitous in a city like New York, which is probably why these crowds wait patiently to be escorted to the center of Atlantic Avenue — once they enter that manhole and climb down the small ladder, they’re entering a living time capsule.

The tours attract a mixed crowd, but a majority of the folks are technology-¬addicted 20- to 30-somethings who sacrifice an hour of no wireless service, no access to Twitter and, evil of all evils, no Facebook to take the tour. When Bob Diamond introduces himself in the dimly lit tunnel, the crowd goes wild; people hoot and cheer. He is a hero. Diamond, a chubby middle-aged man dressed in flannel and wearing a knit cap, is their urban Indiana Jones. Diamond talks about the history of the tunnel, from witty stories of his search for the tunnel to intriguing tales of its historic folklore. He compares the tour to an off-Broadway play. (At one point in time the tour even had costumed actors who re-enacted the true story of an 1844 assassination of an overseer by a sandhog, which was what they called railway workers in those days.) It’s not a far cry, though, considering the tunnel has also been used as a performance/art space.

The real draw, though, is Diamond’s narrated tour. The people on the tour bring flashlights and follow him around as he pauses to tell the stories of the tunnel. The tales are fascinating, rich, engaging and plentiful. In addition to the story of John Wilkes Booth’s missing diary pages, Diamond tells detailed accounts of river pirates who had secret entrances to the tunnel from the old Atlantic Avenue bars, corrupt ¬19th-century politicians, the escapades of the Smoky Hollow Gang, German spies, ghost sightings and many other stories. As you hear the tales in this unchanged tunnel, you can feel the history all around you. When you leave the manhole after the theatrical-like tour, it feels as if you’ve just stepped out of a time machine.

Yet, this play doesn’t have an ending, because there is another 250 feet of tunnel that hasn’t been uncovered. Instead, at the end of the tour, folks crowd around a wall, and some touch it and talk about what they think might be behind it. In recent years there has been serious interest in uncovering it, because, until the wall is removed, the entire history of the tunnel hasn’t (and can’t) be told.

Diamond is excited to finally discover what may or may not be there. “In a very limited way, I sort of found out what Neil Armstrong and Buzz Aldrin felt like when they set foot on the moon, because after we broke through [the first] three-foot-thick concrete wall and got inside the tunnel, it was like landing on a different planet out in space. Going behind [the wall at the end of the tunnel] is going to be repeating all that excitement, but tenfold.”

But will he find the missing pages of the diary like he heard about in The Cosgrove Report? Those missing 18 pages of Booth’s diary have baffled historians for more than a century, and they’ve also been used to fuel creative works like the O’Toole book and the 2007 Hollywood blockbuster National Treasure: Book of Secrets, where the character Ben Gates (played by Nicolas Cage) has to uncover a mystery hidden within the diary’s 18 missing pages. Diamond refers to his search behind the wall as “National Treasure 3.” If The Cosgrove Report was correct about the tunnel being there, then maybe the book is right about the missing pages of the diary also being hidden in the tunnel. According to Diamond, O’Toole wasn’t only a novelist, he was also the CIA’s resident expert on U.S. Civil War–era espionage. O’Toole served with the agency as chief of the Problem Analysis Branch. Maybe he knew some secrets and decided to disguise them as historical fiction? It’s a possibility, but since the author passed away, there is only one way to find out.

Not everyone is sold on the idea that the pages are there, and Diamond has been compared to Geraldo Rivera and his quest for Al Capone’s vaults. Ron Schweiger, a Brooklyn borough historian, hopes “they don’t get the same result that Geraldo Rivera got. I hope there is something there. It won’t just be great for Brooklyn, but it will be a great historical find.” Diamond, for one, believes it won’t be a failed attempt like Rivera’s quest for Al Capone’s vaults. In fact, he seemed quite confident when he claimed, “We are going to find something historically significant.”

There’s also the added question of why the pages would be hidden beneath a New York City street to begin with. Michael W. Kauffman, author of American Brutus: John Wilkes Booth and the Lincoln Conspiracies, admits, “John Wilkes Booth often took trips to New York while he was engaged in the conspiracy against President Lincoln. Those trips were never investigated, and Booth’s New York contacts were apparently never questioned. The government was more interested in proving a Confederate connection than a New York one, so they ignored all leads that pointed northward. I’m very skeptical about the chances of finding those diary pages, but I have to admit that the search looks like an interesting project. One never knows what will turn up.”

WHEN THE WALL is removed, Diamond hopes to find all the answers to the stories of the tunnel that he tells on his tours; the stories that he has thoroughly researched and documented on the nonprofit’s website, www.brooklynrail.net. He doesn’t believe he’ll be disappointed when they reach the other side. No matter what they find, though, Diamond says, “It’s the folklore that makes the tunnel valuable. It’s the folklore that separates this tunnel from any other old railroad tunnel. This is the main artifact of the tunnel itself.”

To tour the tunnel, visit www.brooklynrail.net or call (718) 941-3160. A $15 contribution is suggested; tour dates are posted on the site.