Doors of Durin
The Doors were constructed in cooperation with the artificers of the Elven Kingdom of Hollin, sometime between Second Age 750 and 1500. These were the days before Sauron's dominion in Middle-earth, and the friendship between Elven and Dwarven kingdoms was a rare and special event. During this peaceful time the Doors stood open, allowing unfettered trade, but with the beginning of the War of the Elves and Sauron the Doors were sealed shut. When Khazad-dûm was abandoned in Third Age 1980 the way of opening the Doors was forgotten.
The two greatest craftsmen of the Second Age, the Elf-lord Celebrimbor and the Dwarf Narvi, built the Doors. They were made like a flush door, the jambs invisible to the eye, and matched so perfectly with the mountain rock that when closed the Doors could not be seen. The slabs were made by Narvi out of a grey material stronger than stone, and inlayed by Celebrmbor with ithildin, which can only be seen in starlight and moonlight. When visible, the fine silver-like inlay showed a hammer and anvil (the emblems of Durin), a crown and seven stars, two trees surmounted by crescent moons, and a single star (the emblem of the House of Feanor). The inscription on the archivolt read:
"Ennyn Durin Aran Moria. Pedo Mellon a Minno. Im Narvi hain echant. Celebrimbor o Eregion teithant. I thiw hin."
("The Doors of Durin, Lord of Moria. Speak, friend, and enter. I, Narvi, made them. Celebrimbor of Hollin drew these signs.")
The inscription was a riddle. The answer was a password that would cause the Doors to swing open. In Tolkien's The Fellowship of the Ring, Merry Brandybuck eventually solves the riddle with the word "mellon", which means "friend" in Sindarin. However, in Peter Jackson's movie adaption, Frodo solves the puzzle.