True volcanic craters are formed by magma exploding through the surface of the ground from below. Pseudocraters (also known as rootless cones) are formed by steam explosions as flowing hot lava crosses over a wet surface, such as a swamp, a lake or a pond. As the steam explodes through the lava the lava can build up crater-like forms which appear very similar to real volcanic craters. The difference is that in a pseudocrater there is no conduit through which magma has passed from below the surface.