Gravel

Gravel is a block typically found in naturally occurring pits or underwater. It falls to the most bottom z-coordinate below it if there is no block under it, like Sand. In Alpha, it has a chance to drop Flint once mined, used to start fires with Flint and Steel. Shovels will destroy this block quicker then normal, like Sand and Dirt. Because of Gravel's falling physics, it can be used to make hallways that seal when water touches torches supporting it; see Airlock.

Gravel has been added to caves, and is infamous for being capable of suffocating the player if two blocks of it land on them. For awhile in an old version of singleplayer Creative mode, that had a glitch 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. Minecarts prevent Gravel from falling.