Sponge

Sponge is a block, which is notable for being able to erase water within a 5×5×5 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.

Sponge is only available as a normal building block in Creative mode. However, in Survival mode, it is possible to obtain sponges by placing them in the map in creative mode and loading the file in survival. The sponge functions the same here, as well as in Indev. Sponges do not work with the new dynamic water system currently in Infdev and Alpha (they can be still obtained for decorational use by inventory editing).

History
Sponge is a type of building block added in the update to 0.0.19a client update on June 20th, 2009.

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)

As an anti-griefing measure
Sponge can be used as the corner pillar of a room such that they are invisible from inside, as shown in image below. This can confuse and slow down griefers when attempting to flood an underground development, leaving more time for them to be caught and stopped. In small rooms, this measure, when implemented in every vertex of said space, can stop that room flooding altogether.