Watch the gap: see New York's biggest subway projects take shape

Below the streets of Manhattan, construction is moving forward on two massive transportation projects: one to connect the Long Island Rail Road from Queens to Grand Central, and another to build a much-needed subway along Second Avenue. Both projects began years ago and still have many years to go before they’ll be fully operational, but the Metropolitan Transit Authority has been keeping the lines’ eagerly awaiting commuters updated through Flickr, posting photos of the massive tunnels as work to create them continues.

For the Second Avenue Subway, crews have now excavated about 65 percent of what will eventually be the 86th Street station, reports the New York Post. Farther north at the future 96th Street station, workers are finishing up concrete walls, additional excavation, and the relocation of gas and water pipelines. Almost one-third of the line’s rails have already been received, and they're now being held in storage in the caverns around the 96th Street Station.

Farther south, beneath Grand Central, work continues on the East Side Access project, connecting the terminal to the Long Island Rail Road. Beyond just the necessary tunnels, the crew is now working on constructing a concourse for eventual passengers. The project is expected to be complete in 2019, while the Second Avenue Subway is expected to begin partial operations in just three years. Below, we’ve collected some of the Transit Authority's most breathtaking photos of both projects’ caverns as they looked at the end of last month.