Abstract: An undersea plow is towed by a surface vessel to inscribe a pattern of marks on a marine bottom of questionable soil stability. The plow digs a trench large enough to be seen by sidescan sonar but does not bury itself so far in the soft soil that it cannot be towed at a reasonable rate. The plow is symmetrical about a horizontal line parallel to the path of the trench so that regardless of how the plow comes to rest when lowered, it will still dig the trench. Periodic sidescan surveys detect bottom soil movements by mapping changes in the pattern of marks created by the plow.