Lavacast.

Lavacast is a "mountain" created using water and lava. The main advantage of a lavacast is that, thanks to a small amount of resources (water, lava, and a small staircase (depending on the lavacast type), you get a gigantic mound of cobblestone that can extend for thousands of blocks.

Lavacasts are extremely common at spawn, but they are usually already somewhat damaged:

An example of lavacast at spawn.
An example of lavacast at spawn.

The consequences of the Mushroom operation are visible.