From Dave Brigham:
Brooklyn native Bob Diamond realized every urban explorer's dream when he rediscovered the Atlantic Avenue tunnel in his hometown in 1980. Built in 1844 as part of the Long Island Railroad, the tunnel was sealed shut in 1861. The tunnel was allegedly used in the 1920's to grow mushrooms, and opened in the 1940's so the FBI could hunt for Nazis. But after that, people forgot about it, and efforts to locate the tunnel were fruitless.
Through hard work, determination and smarts, Diamond found the tunnel. He led tours there for 28 years. Then his license to guide people through the tunnel was revoked.
That's the short version. Read Diamond's story here. If you'd prefer, you can watch the 9-minute video that's embedded in the story. I suggest doing both.