Dwarf doors are invisible when closed. Their own masters cannot find them if their secrets are forgotten.
10 seconds sound clip from the The Lord of the Rings: The Fellowship of the Ring movie soundboard.
You can hear this line at 01:58:57 in the Blu-ray version of the movie.
Quote context
[...]
- Evil will be drawn to you from outside the Fellowship. And, I fear, from within.
- You must trust yourself. Trust your own strengths.
- Why doesn't that surprise me?
- Well, let's see. Ithildin. It mirrors only starlight and moonlight.
- It reads, The Doors of Durin, Lord of Moria.
- What do you suppose that means?
[...]