Wikipedia: Atlantic Avenue Tunnel

The Atlantic Avenue Tunnel (or LIRR Tunnel) in downtown Brooklyn is an abandoned subway tunnel built in 1844, the oldest in the world, running beneath Atlantic Avenue for about 2750 feet between Hicks Street and Boerum Place. It had been sealed in 1861, fell into myth, and only rediscovered by the 18-year-old Robert "Bob" Diamond in 1981, who entered from a manhole at Atlantic and Court Street.

It is listed on the National Register of Historic Places since 1989.

Tours of the tunnel had been held, led by Bob Diamond, until October 2001, when the financial collapse of the BHRA forced Diamond to stop them. [1]

Walt Whitman wrote of the tunnel:

: The old tunnel, that used to lie there under ground, a passage of Acheron-like solemnity and darkness, now all closed and filled up, and soon to be utterly forgotten, with all its reminiscences; however, there will, for a few years yet be many dear ones, to not a few Brooklynites, New Yorkers, and promiscuous crowds besides. For it was here you started to go down the island, in summer. For years, it was confidently counted on that this spot, and the railroad of which it was the terminus, were going to prove the permanent seat of business and wealth that belong to such enterprises. But its glory, after enduring in great splendor for a season, has now vanished - at least its Long Island Railroad glory has. The tunnel: dark as the grave, cold, damp, and silent. How beautiful look earth and heaven again, as we emerge from the gloom! It might not be unprofitable, now and then, to send us mortals - the dissatisfied ones, at least, and that's a large proportion - into some tunnel of several days' journey. We'd perhaps grumble less, afterward, at God's handiwork.

History

In March 1916, the FBI suspected German terrorists were making bombs in the tunnel, and broke through. They found nothing, installed an electric light, and resealed the tunnel.