This is a $13 million tunnel boring machine (TBM) used for tunneling at the Nevada Test Site. (Remember that Area 51 is part of the test site.) Many other types of TBMs are used by many govt. agencies, including the "nuclear powered TBM" that melts solid rock and leaves behind glass-like walls. Most tunneling activity is under military installations and all information is highly restricted.
Former employees of said facilities have surfaced over the years to talk of massive underground installations in places like Area 51, Northrop Facility in Antelope Valley, Ca.(rumored to have 42 levels), and the Lockheed installation near Edwards, Ca.