Sources: Nature [journal paper], Science [journal paper], University of Alberta [press release], The Guardian
quote: The deepest borehole was done by the Russians to 40,230 feet (7.6 miles).The Earth's crust is between 20 to 30 miles deep.Good luck with that.
quote: Several interesting discoveries resulted from the hole. Pockets of water were found from time to time, prevented from ascending to the surface by a layer of impermeable rock. An excess of hydrogen was released from the drilling, which researchers described as "boiling" up from the surface of the hole.