(1) The explosion of an artillery shell, mine, or aerial bomb underground without forming a crater; it is sometimes detected by the underground sound of the explosion, by the slight swelling of the ground above the point of the explosion, or by smoke escaping to the surface through cracks.
(2) An underground explosion especially arranged to destroy enemy underground structures.
(3) In a metaphorical sense—an unexpected unpleasantness, a dirty trick, a failure.