Sitting below one of the most famous parks in the world is a half-mile train tunnel built by the city of New York forty years ago that you won’t find on any subway map. Sometimes it’s there, and sometimes, it’s not.

This tunnel has sat below Central Park, barely used, for decades. Thanks to the first major piece of work done to New York’s subway system since the 1980s (paywall), commuters will soon be able to ride in this previously hidden subway structure.

The tunnel, which runs between the 57th Street and 7th Avenue, and Lexington Avenue and 63rd Street in Manhattan, has never had a dedicated train line running through it. If you’re heading northbound on an N, Q, or R train from 57th Street, for a second, you’ll get a glimpse of it, but according to archivists for the Metropolitan Transportation Authority (MTA), New York’s governing body of public transit, it’s only appeared on subway maps for two brief periods in 1995 and 1998.

The tunnel was built in the early 1970s, as part of a connection between Manhattan and Queens along 63rd Street. The extra loop of tunnel was originally part of a project to create a new subway line along Second Avenue, but this plan was abandoned when a recession hit the city in the 1970s. All these years later, the city is finally getting around to completing that Second Avenue line.

Assuming everything goes according to plan, commuters in New York City will soon be traveling through the piece of tunnel below Central Park. The MTA is in the process of completing the Second Avenue line, nearly a century after it was first proposed.

The first step by the end of December will be to reroute one of the MTA’s existing train lines, the Q train, to run along part of the new Second Avenue tunnel in northeast Manhattan, instead of to its current destination in Astoria, Queens. In a document outlining the rerouting of the Q train to Manhattan last month, the MTA released a preliminary version of what the subway map would look like with the Q line running through Manhattan, and a new W train running to Queens:

Metropolitan Transportation Authority Revealed.

Finding out about this secret tunnel required a bit of detective work. This is where my suspicions were first raised, as currently the map looks like this:

Metropolitan Transportation Authority Today.

I called a spokesperson for the MTA to ask where this extra loop was coming from, and whether it was a new construction. I felt like people would have noticed if the MTA had been ripping up Central Park to build a tunnel. He told me that it was not new, and that it had indeed appeared on maps before, but couldn’t point to any specific instances of when or in what capacity.

Wanting to know more, I asked the archives department of the New York Transit Museum, which is affiliated with the MTA, if they knew of any instances where this ghost tunnel had appeared on any maps. Halley Choiniere, an archivist at the museum, looked through every instance of the New York subway map that the MTA had given the museum since the tunnel was first contracted to be built in 1970. She was found just two periods, of about six months each, in the summer of 1995 and the winter of 1998, where the tunnel appeared on the map.

In 1995, the mysterious tunnel was included on the map when the Manhattan bridge was out of service, allowing Q trains to cross back over to Long Island farther up the East River while the bridge was being worked on:

Courtesy of the New York Transit Museum The rerouted Q train in 1995.

Once the work was completed in late 1995, the tunnel disappeared, and the Q train went back to its regular route.

In 1998, the tunnel reappeared as a special temporary shuttle service while work was being done on the Sixth Avenue line, cutting off access to lower Astoria through the regular route:

Courtesy of the New York Transit Museum The special “S” train in 1998.

Again, when the work was finished, the tunnel disappeared, and the map went back to its regular delineations.

The former ghost tunnel will now appear on the map full time, as the Q train will now run through it on its way across the city and up Second Avenue. The MTA wasn’t able to confirm why the tunnel isn’t usually listed on the map, other than the fact that it is so rarely been in use over the past 40 years.

It does lead one to wonder if there are other undocumented underpasses lying beneath the streets of New York.