A hole, and people die if they fall in there.
Explanation:
A wavefront is the long edge that moves, for example, the crest or the trough. Each point on the wavefront emits a semicircular wave that moves at the propagation speed v. These are drawn at a time t later, so that they have moved a distance s = vt.
The solution is 'supersaturated' ... holding more solute than it's supposed to be able to hold.
