The Second Avenue Subway is more than 80 years in the making. Some said it would never be done. Yet, deep underneath Manhattan this spring, the final framework is being laid for a system that will carry millions of commuters through the city—and it looks downright primordial.

The MTA sent photographer Patrick Cashin into the bowels of the Second Avenue subway a few days ago to document the superstructure of steel and concrete that's rapidly filling the voids below the city. At 72nd Street, a massive steel platform has a hole cut out where, we can guess, the escalators might eventually go:

Meanwhile, at the 86th Street station, the tunnel is prepped with rebar:

A different section of the tunnel shows what comes next. Here, we see what look to be pre-cast concrete panels, forming a clean surface:

The elevators used by workers to descend into the deep construction site glow with orange safety paint:

Some spaces are so thick with structure and detail, they look like they're filled with thick steel spider webs:

To me, these photos are even stranger, somehow, than the empty caverns we saw just a few weeks ago. Check out many more over on the MTA's Flickr page. [MTA on Flickr]