This may be under Atlantic Avenue.
"With continued silence from the DOT, we are dead in the water—with the potential of a major historical find right under our feet in Brooklyn."
Earlier this month, Bob Diamond—who is responsible for rediscovering the old Atlantic Avenue Tunnel back in the 1980s (where he was allowed to give tours until the FDNY shut him down)—shared a series of internal emails between employees at the National Geographic Channel. The above sentence is pulled from the channel's former VP of development, Bill Margol, and hits the root of the issue. There is something of historical significance under a street in Brooklyn, which experts could easily get to, but it's been enshrouded in red tape.
A few years ago, the Nat Geo team started working on a project in which they hoped to film the big reveal of an old 1830s train believed to be underneath Atlantic Avenue, in an unexplored and currently inaccessible part of the tunnel Diamond had found. Other things that could be down there include John Wilkes Booth’s diary (which may "reveal who hired him to shoot Abraham Lincoln")—historical novelist G.J.A. O’Toole wrote in his 1979 detective thriller The Cosgrove Report that "the missing pages of the assassin’s journal were hidden in a steel box under Atlantic Avenue. Next to a lost steam locomotive."
So the Nat Geo program was going to be like when Geraldo Rivera opened Al Capone's vaults... but more successful. But you've never seen this program because almost immediately after they all but confirmed the locomotive's existence, the NYC DOT shut them down.

The magnetometer image of the locomotive. (Courtesy of Bob Diamond)
The channel's Bill Margol also wrote in the aforementioned emails: "We found a signature of a large metal object, about the size of a locomotive, exactly where our sources and historical information predicted it would be. As you can imagine, this was an amazing find and word quickly leaked to the press. This angered Seth Solomonow at DOT and he effectively shut down all access to the tunnel, at which point our production was officially put on hold." That was in 2011, and there has been no progress since, except for Diamond's fruitless attempts at cutting through the red tape.
The reason the mystery remains a mystery seems to come down to a series of grudges. The DOT has never liked Bob Diamond much, and Margol at Nat Geo called Diamond "mercurial, at best" in these emails. Diamond, who recently filed a lawsuit against Nat Geo, sent out the emails to press, declaring that they contain a "full written confession from Bill Margol admit[ing] Nat Geo caused DOT to shut down our tunnel." He told us today that his lawsuit against them "has served to get their attention. They've indicated through their attorneys, that they are willing to settle with us. They have also mentioned that they feel bad, because they already spent over $100,000 on a show that never came off." Diamond is "requesting reasonable compensation for loss of tunnel tour income for the last 5 years, and that Nat Geo join with BHRA to re-approach DOT, unveil the locomotive to the public, help underwrite the cost of improving access to the tunnel, and finish the show."
Diamond tells us he's "written numerous letters to Polly Trottenburg [DOT] and Mayor de Blasio asking for a meeting to try and get this project back on the rails. The only answer we ever got, is a form letter generated by the mail office at DOT." The letter basically says: no comment. He added that "the only thing stopping us from going back into the tunnel is the DOT. Of course, we're glad to also work with FDNY to address any safety concerns they may have, regarding the tour groups. In 2011, our professionally licensed civil engineer prepared a plan for greatly improving access and egress to the tunnel, which wouldn't cost too much money. At the time, we submitted the plan to the DOT, but never got an answer."
According to the Daily News, "Department of Transportation PR man Seth Solomonow pulled the plug on the project in part because leaks in the press and Diamond’s lawsuit cast the agency as the bad guy.” Noting that in a 2011 discussion with Nat Geo's Pam Wells, Solomonow declared, "If people from the Channel want to call me they'll simply be hearing me ‘yell at them’ and then (I’ll) make sure the film will NEVER HAPPEN... With the DOT being cast as the bad guy by Bob Diamond, we cannot, at this point, allow National Geographic access to the tunnel...Let me make it clearer. Do not call us. We will call you.”
We've reached out to Solomonow for comment, but received a reply saying, "I am no longer with DOT." We've also reached out to current employees at the DOT, and will update when we hear back.
Here's an old History Channel piece on the tunnel, showing Diamond in the tunnel:

