East Side Access tunnel that has been carved out more than 100 feet below under Grand Central Station and Park Avenue over the last 4 years. The machine that did this incredible job is a huge TBM (tunnel boring machine) which, now that the work is done, will be buried below the intersection of 37th and Park because it's basically too expensive to extract it out of the tunnel. That's right...they are just going to leave it there encased in cement, with a nice plaque of course, to explain this weird subterranean artifact to future generations who might discover it in the future.
This remarkable story was in the TIMES earlier this week. Apparently this sort of machine burial is often done on tunneling projects due to the expense of removing the machine. However, it's never been done under New York because the underground real estate is often just as precious as it's above ground counterpart. But in the case of this East Side tunnel, removal would not only have been ridiculously expensive (about $9M!) it would have been impossible to open up a 40 foot hole in the middle of the tony block which boasts the Union Club.
Anyway, this is another fascinating story of what's going on underneath the city as part of the largest expansion of the transit system in more than 60 years. East Side Access trains are set to run in 2016.