Trapdoor

The Trapdoor (also known as Hatch) is a block used as a horizontal door that fills one block space. It was introduced in Beta 1.6.

They are placed over a single-block hole (but can be placed over a solid block as well) with a solid block directly adjacent. When activated, they will then open by flipping upwards toward the adjacent block. If that block is destroyed, the trapdoor is as well. Due to this restriction it is currently not possible to create a flat floor with a trapdoor opening downwards.

When placed outside, rainfall, water and snowfall do not pass through a trapdoor, but light is allowed to shine through.

Usage
As with Doors, Trapdoors can be controlled with Redstone circuitry. A charge will cause the trapdoor to change its state, from either on or off, depending on its starting state.

It's also possible to create a distributor with a vertical succession of trapdoor linked with redstone.

See Tutorials/Trapdoor Uses for Tutorials/Examples.

Bugs

 * Trapdoors block movement when approaching from another block on the same level and the trapdoor has a block exactly 2 spaces above it, but it does not block movement in the same case if approaching from another trapdoor, or if you are approaching the trapdoor from a space that is also 2 blocks high. This allows one way drawbridges.
 * If there is a ladder beneath a trapdoor, you can climb down, even if the trapdoor is closed.
 * If you save the game on a trap door and then join back it puts you in the nearest block.
 * Trapdoor textures are mirrored incorrectly.
 * Trapdoors (at least at the top of ladders) do not stop mobs from interrupting your sleep. Maybe caused because of the bug that allows you sometimes to go through closed trapdoor on top of the ladders without opening it.
 * Trapdoors can only be put on a block higher than the one on the surface, not one in the ground.

Trivia

 * Sand and Gravel, if dropped onto a trapdoor will break into items (similar to how sand or gravel falling on a torch will do the same).
 * Liquids (water and lava) cannot fall through a single hole trapdoor. The trapdoor counts as a block and can't have something else in it, even in its open state.  Trapdoors will stop the block above it whether it's open or closed.
 * Trapdoors will not prevent you from accessing the contents of a chest if the trapdoor is in the closed state directly above it. This means that they are just like any other Door and allow you to interact with objects on the other side even if the trapdoor is closed.
 * If you put a trapdoor on a lava block, the lava will disappear and the trapdoor will not be burned. This allows the creation of paths.
 * Trapdoors can be used as ladders by placing them one above each other down a hole. Then from the top hit them to open them to go down then close one at a time and jump to go back up.
 * The drop version of a trapdoor is much bigger than other drops.
 * Trapdoors can be used as fuel in a furnace but burn only as much as a wooden plank.

Videos
A basic trap built with Trapdoors