Lavacasts are player-created structures often used for griefing. Lavacasts are also one of the main reasons for the collosal stone towers in spawn. They act as terrain for new players to travel across.
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, the most famous example of this is the giant cobblestone swastika . Lavacasts are never intentionally placed in serious, aesthetical bases, but are common as a cheap way to do things.
Using lavacasts to grief a build is called "lavacasting" (for example: "I have just lavacasted The Valley of Wheat for the 326th time!!").