Gravel

Gravel is a common block on new maps, it is found underwater. Like sand 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 can be used in conjunction with torches to create self-sealing hallways; See Airlock

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.