Having been inspired by Jules Verne's Journey to the Center of the Earth, Emmett Brown developed the idea of building a rocket-powered drill. He sent in a patent for it at the patent office, but never heard back, resulting in Emmett not pursuing the idea any further. That changed when Marty McFly traveled to June 13, 1931 to free the older Doc Brown from prison and needed the drill to do so. Marty approached the residential Emmett Brown to build it, and managed to convince him he was a scientist, though Emmett also figured he was from the patent office.