They were wrote by dwarves but you cannot see them
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The moon letters brought out additional information. Elrond found them while looking at the map with Gandalf and Thorin one evening. They provided information on the secret passage.
-T:Prophesied what? -Prophecied when the keyhole to the secret entrance of Lonely Mountain would be visible. -T:Well the moon runes on Thráin's map told them. Also, that's not a prohecy because they made the door to be that way themselves.
The book by J.J.R. Tolken shows Bilbo and Thorin and Co. using a key, Bilbo's sword, Sting, and, of course, the ring. One other thing that is sort-of apart from the map is the fact that the all-important moon-letters, which were found by Elrond the elf.
Durin's Day indicates that the dwarves can open the door with the key on the last moon of Autumn.
The map was very old and showed a secret passageway to the mountain.