i know there are many ancient pre, pre great flood tunnels intercontinental worldwide of humungous height and width………
As far as current technology…The average depth of these bases is over a mile, and they again are basically whole cities underground. They all are between 2.66 and 4.25 cubic miles in size. They have laser-drilling machines that can drill a tunnel seven miles long in one day. I was involved in building an addition to the deep underground military base at Dulce, which is probably the deepest base. It goes down seven levels and over 2.5 miles deep. I helped hollow out more than 13 deep underground military bases in the United States. Presently, there are 129 deep underground military bases in the United States
Many types of TBMs are used by many government agencies, including the ‘nuclear powered TBM’ [NTBM] that melts solid rock and leaves behind glass-like walls
http://www.sheepletv.com/nuclear-tunnel-boring-machines-switzerland-has-nothing-on-us-2/