IT COSTS AN estimated $4.5 billion (€3.4 billion) and it has taken 90 years to get off the ground – or at least, under it-  but New York City’s Second Avenue Subway is slowly but surely taking shape.

Photographs released by the Metropolitan Transportation Authority (MTA) in New York show the massive construction project taking place under the city’s streets, which is the first underground line to be built in the city since 1932.

The Second Avenue subway has been a long time in the planning – Mad Men even made a joke about it in a recent episode, with one estate agent telling Peggy Olson, “Believe me, when they finish the Second Avenue subway this apartment will quadruple in value.” (The episode was set in 1968).

MTA photographer Patrick Cashin has been documenting the construction for the past four years, beginning with when giant eight-storey-high tunnels were excavated on the city’s Upper East Side.

“When I arrived on the scene in 2009, it was just a lot of mud and dirt,” Cashin said in a video on the MTA’s Flickr page. “But as I kept going back, this hole kept getting deeper and deeper and soon it extended several blocks.”

There are an estimated 800 people working on the construction site, and Cashin visits the site every few months to update the progress.

“When you’re down there for all of 10 seconds, you know that this is a dangerous place to be,” says Cashin.

“What’s really impressive is when you walk through where the tunnel boring machine had cut the 22-foot hole,” he says.

And then boom, you’re in this huge cavern. It’s this huge hole where the 72nd Street Station is going to be, and it just hits you how big, how much digging they had to do to get this cavern made. It’s just amazing.

The subway line is currently schedule to open in December 2016.

(All photos: MTAPhotos/Flickr/Creative Commons)

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