Doc and Marty blast a hole in the sealed entrance.


The DeLorean inside the mine.

"Take this camera. I want to document everything!"
—Doc Brown
"There, at the center of the new hole [blasted by Doc and Marty using dynamite], was a tunnel, leading into the hill and down. As they got closer, Marty could see it was a real mining tunnel, the ceiling supported by what looked like old railroad ties, with a pair of narrow railroad tracks running down into the darkness."
—From Back to the Future Part III by Craig Shaw Gardner (quote, page 18)

The Delgado Mine was a mine located next to the Boot Hill Cemetery. It was where Emmett Brown, who was unable to fix the badly damaged DeLorean time machine due to lack of suitable replacement parts in 1885, hid the car for seventy years. Doc sent a letter to Marty in 1955 via several generations of Western Union to inform him of this, and also put an envelope containing repair instructions, alongside a schematic diagram, in the DeLorean to enable Marty and his younger self to restore it to working order using 1955 components to replace the burnt-out time circuit control microchip.

Marty and Doc discovered the DeLorean shrouded in a cloth within a murky and dark cavern behind a wall with Doc's initials on it. While analyzing the parts that failed when it was struck by lightning, Doc claimed that the problem was the source of the part, Japan — to which Marty said that all the best stuff was made in Japan (at least in 1985).