A wall cloud is a sizeable structure, often rotating.
Storm spotters are often sent out specifically to keep an eye on rotating
wall clouds that have been detected by radar or the public. The wall cloud
literally hangs beneath the base of the thunderhead, and it's usually very
dark. Its edges are fairly well defined and it may trail a "tail" that
extends out and slightly downward; this is not a tornado but part of the
wall cloud itself. A funnel often forms from the bottom of a wall cloud.