Sponge

Sponge is a type of building block added in the update to 0.0.19a client update on June 20th, 2009. Sponge's main properties that separate it from other building materials is sponge erases water within a five by five area, with the sponge block at its center. Because of this, sponge is an ideal material for unflooding caverns and keeping water out of tunnels. Players should keep in mind that the way the sponge works is just a temporary hack until finite water comes out.

Properties
Sponge is not affected by gravity, unlike sand, and it will not fall if a block is deleted underneath it. Sponges do not need light to survive, and do not decay (sponges used to decay in the implementation of the 0.0.19a update, but this was changed within 50 minutes of the release)