Lavacasts are enormous cobblestone structures, frequently used on 2b2t as means of griefing. While they are ridiculously easy to make, using only water and lava, a lavacast exponentially grows in size as it is being made.
A lavacast usually comes in the shape of a pyramid, made from many layers of cobblestone. They rely on the generation of cobblestone when water and lava collide. Players first let lava flow over the lavacast, and once it reaches the bottom they empty out a water bucket on top, which flows down and turns all of the lava into cobblestone.
There are three types of lavacasts: pillars, walls and cobble monsters. Pillars are the easiest to make, they are pillars of lava cooled down with water to make stone pillars. Walls are the most iconic type, they look like walls or stairs. They are great for griefing. Now for the last, cobble monsters. Cobble monsters take the form of mountains and pyramids and are often found in places that have been griefed to oblivion, like 2k2k and 200 150.
A video showcasing the process of creating a pyramid-shaped lavacast:
A video showcasing a staircase-shaped lavacast (these are easier and faster to make, helpful for when you have limited time.)