Trapdoor

The trapdoor is a block item implemented in the beta 1.6 update.

They are intended to be 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. Just like doors, trapdoors can be activated by redstone circuitry. When placed outside, rainfall, water and snowfall do not pass through a trapdoor, but light is let in through the four gaps on its surface.

Crafting
When crafted with six wooden planks, two trapdoors are created. Video



Redstone
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 it starting state.

Drawbridge
If you place the trapdoors in a trench in this fashion:

You can create a fairly good drawbridge. You can extend it lengthways, but not widthways. However, it's possible to walk on the ledge of an opened trapdoor if there is enough space above it.

Bugs
Trapdoors seem to break faster using a pickaxe instead of an axe, like wooden slabs.

Trivia

 * Much like the Piston is based on a mod developed by Hippoplatimus, the hatch is very likely based on a mod from DJoslin. (http://www.djoslin.info/trapdoor.php)
 * Trapdoors may not be placed next to double half blocks.
 * 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 it's open state.  Trapdoors will stop the block above it whether it's open or closed.