Gravel

Gravel is a common block on new maps, it is found underwater. Like sand, it is also affected by gravity and will fall to the lowest solid block in the z-coordinate it is placed in. Mining it can drop flint as of 2/19/2010 in indev. Gravel originally only appeared under pools of water. In recent game modes, it now spawns in patches above ground too. As of June 17, 2010 patches of gravel can now be found underground and in caves.

Gravel can be used in conjunction with torches to create self-sealing hallways; See Airlock

If placed on top of a Minecart, the bottom layer of gravel will not fall once the minecart is removed.

History
A glitch was present in Single Player for quite some time that caused water to flood from blocks affected by gravity if placed against a wall but not on the ground. The bug has since been fixed.