Soul Sand

Slow sand (aka "mud", "hell mud") is a brown-grey dirt type block, found in The Nether. This material has the most of same properties as Dirt except it does not have grass and slows movement of any player or NPC's speed. They also sink slightly into the block if they walk on it.

Trivia
The texture resembles a collage of faces. These faces are said to look like those of Creepers.

This block stopped generated in the v1.2.0_01 update; in its place, gravel was generated. This was fixed in the v1.2.0_02 update.

The first appearance of this block was on Notch's blog.