Lava

Lava is a substance only found underground in pools. It can be almost anywhere underground on a newly generated map.

Like water, lava spreads by duplicating itself to open horizontal and downwards squares. Lava is quite slow though and can be easily outrun. If quick enough a skilled player can block the flow of lava. If the lava wave is large enough though, a player will not be able to build fast enough. Also like water, Lava slows down the player moving through it but it does so to a greater extent. Lava is also more opaque than water and is harder to see through. If water and lava collide, a block of Stone is created. This 'technique' can be used to manipulate water and lava.

Lava's texture was changed in the 0.0.19a client update on June 20th, 2009, and is now animated.