- "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.
The mine was where Dr. Emmett Brown, who was unable to fix the badly damaged DeLorean time machine due to lack of suitable replacement parts in 1885, had hidden 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 rolled-up schematic diagram, in the DeLorean to enable Marty and his younger self to restore it to working order using 1955 components — including vacuum tubes — to replace the burnt-out time circuit control microchip.
Because the mine was sealed, Doc had to blow it open using dynamite so he and Marty could enter the shaft. On the way down, Doc told Marty that the tunnel reminded him of Journey to the Center of the Earth, a book he had read as a boy and which inspired him to become a scientist.
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 the car 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).
After Marty traveled back to 1885, he and Doc camped out next to the mine on September 6 as they prepared to return to the future. As per Doc's plan, they released the DeLorean onto the railroad tracks in front of the mine that night and it was here their journey back to 1985 began after 'borrowing' the train and bringing it to the DeLorean.