Doors of Durin
The Doors of Durin were created in the dark cliffs of Moria, in the cooperation of the Elves and the Dwarves before Sauron's dominance in Middle-earth. In these times, they stood open and were guarded by a doorwarden, allowing free and friendly trade between the Elves and the Dwarves; but by the Third Age they had been closed on the world; unable to be found if their secret is forgotten.
They can only be found if one who knows their ancient secrets passes his hands over the rocks. The cliff is made of ithildin, which mirrors only starlight and moonlight. When the moon is out in full, fine silver lines appear, outlining the secret door. The designs on the arch include a hammer and an anvil (the emblems of Durin); a crown and seven stars; two trees surmounted by crescent moons; and a single star (the Star of the House of Feanor). The inscription at the top of the arch reads: "The Doors of Durin, Lord of Moria. Speak, friend, and enter.""
In the novel by J.R.R. Tolkien, it is Merry Brandybuck who eventually solves the riddle (the password is "Mellon", the Elvish word for 'Friend'). However in Peter Jackson's movie version of The Fellowship of the Ring, it is Frodo who solves the puzzle.
To see Tolkien's drawing of the Doors of Durin, refer to Chapter 4 in Book 2 of the Fellowship of the Ring - A Journey in the Dark.