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 retards movement of any player or NPC's speed.

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

This block no longer generates in the v1.2.0_01 update. It still exists in areas in the Nether where it already has generated, but in further areas, it is replaced with gravel.