The Atlantic Avenue tunnel, after vanishing into local mythology
in the 1860s, was first rediscovered in 1916 by a group of guys hunting for German terrorists
The legend of the tunnel was never entirely dead. Various New York City newspapers, like the Brooklyn Eagle, would mention it now and again. Rumor was that the tunnel could be accessed via a secret door in the basement of some unknown Atlantic Avenue bar. Periodic bar crawls cum exploratory parties, however, never found any such portal.
Some fellows with good heads for secrets and well developed senses of paranoia decided that the phantom tunnel under Atlantic Avenue just might be harboring Germans. They worried that these Germans just might be mixing up a batch of mustard gas. So they set out to calm their fears.
They found the tunnel. Mere yards from the manhole which would soon be revealed as the tunnel's secret entrance, they broke through the street, dug through packed dirt, and eventually found brick. A bit more smashing and digging, and the edges of the ragged hole collapsed away into darkness. Cool air rushed out of the ground at them.
Down they went.
The tunnel they discovered was approximately seventeen feet high; the upper three quarters were brick, the lower quarter Manhattan bluestone (quarried from the present site of the United Nations). The earth floor was four stories below street level. Old soot was caked onto the ceiling and walls. No tracks, or railroad ties; those had vanished to scrapyards or other railways seventy years earlier.
No Huns were crouching in the gloom, alchemizing chlorinated death for Brooklyn.
Our explorers returned several times. They strung up electric lights. One of them attempted to immortalize himself and his deed ("hung the first electric light in the subway, 1916") with white paint applied to the bluestone near where they entered. The paint is partially flaked away now. I assisted its erosion a bit by tracing the "signature" with my finger. His colleagues added their names or initials and reiterated the date.
Their last deed was to seal the end of the tunnel nearest the manhole with concrete. Krauts would never make this a secret base. The concrete would remain inviolate until Bob Diamond re-rediscovered the tunnel decades later.
You can, for $15, take a guided (by Bob Diamond himself) tour of the Atlantic Avenue tunnel. I say it's well worth it. Tours are given on several weekends scattered throughout the year. All the money benefits the Brooklyn Historical Railway Association, and furthers Diamond's dream of excavating the remainder of the tunnel and putting it back into service as a part of a proposed trolley system. You also have the option of volunteering when Diamond and his associates excavate the station buried at the Hicks Street end of the tunnel. There is, Diamond claims, a complete locomotive from the 1830's buried back there.