In February, the city released literally awesome photos of a massive cutter head for a 1000-ton tunnel boring machine, as it was lowered into a "launch chamber" hole near the intersection of 25th Street and 11th Avenue. The tunnel boring machine, or TBM, has been working its way east since then, as part of a $2.1 billion project to extend the 7 subway line from Times Square to the Hudson Yards. Now the mayor's office has dropped this fresh video of the TBM cutter head breaking through the 34th Street Station Cavern Wall. The fun starts at the 30 second mark, and climaxes when [SPOILER!] triumphant workers crawl through the cutter head from the other side:

The excavation breakthrough happened on December 14th, and marks the completion of the first phase of the project. "Too often, government falls victim to the temptation to abandon long-term infrastructure projects amidst short-term downturns, and that’s why big things never get done," said Mayor Bloomberg in a statement. (Today, the City Council is expected to approve a major rezoning plan for the Hudson Rail Yards, which would turn the area into residential and commercial towers served by the 7 train.) According to a statement from the MTA:

The first 300 feet of tunneling was complicated by a section of soft ground between 27th and 28th Streets that required a technique called 'ground freeze' to reinforce the ground, allowing the machines to pass through as if it were solid rock. As the tunnel boring machines mine, they place pre-cast concrete lining rings along the excavated tunnel, making up the permanent liner of the finished tunnel.

Tunneling north from 34th Street also presents unique challenges, as track will run under the 8th Avenue Subway, Amtrak/NJ TRANSIT tunnels, tunnels to the former New York Central Line, the Lincoln Tunnel and the Port Authority Bus Terminal and ramps. Excavation and underpinning of the 8th Avenue Subway line is underway to allow the new tunnels to tie into the existing 7 Line tail tracks at Times Square.

Tunneling is expected to be completed in the Spring of 2010, when work will commence on station entrances and finishes, as well as support facilities such as ventilation and traction power substations. Extended 7 train service is expected to begin in December in 2013.