Soul Sand

Soul Sand (also known as Slow sand, Quick sand, Mud, Hell mud or Nethermud) is a brown-grey dirt-type block, only naturally found in The Nether, usually near large bodies of lava in deposits similar to gravel deposits. It is commonly mistaken that the best tool for harvesting soul sand is to be a shovel because harvesting it makes the same sound as dirt, gravel and sand. However, shovels make no effect in speeding up the process. Swords can harvest it slightly faster, but this costs two uses of the sword instead of one.

This material has looks somewhat similar to a Dirt block, but it has some unique features. It slows the movement of any player or mob, and players sink slightly into the block while walking on it. It is only 15/16 the height of a normal block, so the player cannot walk from a block of soul sand to a slab without jumping. This also causes that you can not get off of soul sand while holding down the sneak key, unless the player jumps.

Uses
This slowing effect is much greater when the Soul Sand block is placed in water and is useful for making drowning traps more effective against mobs. This block will also prevent water currents from transporting items over it. For example, used in a cannon to stop water from moving the TNT. When placed directly over an ice block, soul sand's slow effect is greatly increased. This affects both mobs and players. The slowing effect can also be used to fight large amounts of mobs at once, especially using a bow.

Soul sand also suppresses the impact from boats when colliding to it at the angle of approximately 6° to 174°. This makes it an effective building material when creating a dock.

One-way and Minecart-only gates


This Video demonstrates both of these uses:

When placed on the ground in front of a two-block high doorway, soul sand prevents players from going through it from that direction, making it useful for building one-way passages or invisible barriers. This affects mobs too as they have to jump to get out of the slowing effects of the block, thus are unable to get through the door. However, it is possible to avoid the sand trap by jumping over the soul sand and squeezing into the one way door. Also, if you keep jumping when trying to go through the entry way, you will get through. Since you do not sink into the soul sand in this case, it is like jumping over an empty space. It is possible for a mob to avoid the trap in this way, but unlikely, since a mob usually only jumps when there is a block one level higher than the mob. A good way to overcome this problem is to simply expand the soul sand so it covers a larger area, rather than one brick. Having soul sand extend three bricks from the doorway, for example, or building a wall just behind it will prevent this. Mobs can also enter the doorway by simply entering diagonally.

Because minecart tracks can be laid on Soul Sand like most other block materials, this quirk can be exploited to create minecart-only gates (by placing soul sand on both sides of the doorway). This is one of the few ways to allow minecart tracks to go directly inside a secure area without risking wandering mobs getting in.

Using the same idea underground, one can create a tunnel trap for wandering mobs and players. By digging out one block of the ceiling of a 1x2 tunnel and placing soul sand underneath, players will be unable to easily leave the soul sand block without destroying blocks.

BUG: If you jump you can still walk through the door. But if you place more than 3 you can't jump through it!

Trivia

 * The texture resembles a collage of faces. This could explain the "soul" in soul sand.
 * The first appearance of this block was on Notch's blog.
 * There is currently a bug where falling off this block directly into lava or water will make the player move slower in said lava/water until you are back on normal land.
 * When first released, Sneaking on a soul sand block would not prevent the player from falling off the block. This bug was fixed in the 1.2.1 Alpha patch.