A way of Minecraft griefing involving building a staircase of blocks and placing a lava source block on the top, letting it flow down the staircase, then placing water on the lava source right afterwards to generate a cobblestone staircase.
Lavacasts mostly consist of cobblestone. They might also contain obsidian (mostly in the center of the cast) and stone (usually on the base, due to bodies of water getting in contact with lava). Of course, anything that has been covered by a lavacast or was left in it can also be there if it wasn't destroyed. They usually resemble mountains or pyramids, and sometimes also spires or other shapes, depending on the terrain and the way the lavacast was casted. Lavacasts often contain small air pockets.
Making and destroying lavacasts
While there are multiple techniques of creating lavacasts, they all generally revolve around pointing water on lava from above. When it comes to destruction, the only two reasonable ways are fast pickaxes (like ones used for highways) and TNT. Even with those, however, making lavacasts is faster than destroying them.
Lavacasts are usually used to grief bases or the spawn. They might also be used to easily create walls, pyramids or artificial mountains. A rare but well-known usage of lavacasts is creating huge symbols. Lavacasts are never intentionally placed in serious, aesthetical bases, but are common as a cheap way to do things.