Hot off the press release:

The Metropolitan Transportation Authority (MTA) today announced that after just eight months, the first of two 200-ton tunnel boring machines had completed its mile-long plus journey from the bedrock beneath the intersection of 63rd Street and Second Avenue to the terminus of what will become a new station and concourse underneath Grand Central Terminal. The second machine is scheduled to complete its parallel journey near the end of the summer.
…
Now that the machine has reached its destination, excavation will begin on what will become a cavern underneath Park Avenue between 49th and 51st Streets that will connect the newly built tunnel with parallel tunnels which will allow the future Long Island Rail Road flexibility in accessing all eight tracks in the new station under Grand Central. That work involves intermittent blasting and mechanical excavation that is scheduled to begin in mid-July and last for six to eight months.

I very much like the idea of tunneling a cavern underneath the existing Grand Central Terminal. I imagine commuters-cum-spelunkers entering and exciting Grand Central on their way to and from home.
Read more about MTA’s East SideAccess.