Dwarven Door

Dwarven doors are invisible to the eye, except in moonlight. These doors emit a glow at night and will become visible when the player is within seven blocks from the door (at night of course).

Dwarven doors spawn naturally in the Iron Hills and Blue Mountains as an entrance to a Dwarven house. Opening a Dwarven door gives the achievement "Mellon!". They cannot be opened by any mob.

Crafting
These doors can be crafted on the Dwarven crafting table or the Blue Mountains crafting table by putting six stone blocks in a 2x3 vertical rectangle (see below).