Final answer:
Vessels engaged in mine clearance operations display three all-round green lights or three balls to indicate it is dangerous to approach within 1000 meters, as part of the COLREGs for marine safety.
Step-by-step explanation:
The lights and shapes for a vessel when engaged in mine clearance operations are specified in the International Regulations for Preventing Collisions at Sea (COLREGs). These include the display of three all-round green lights or three balls. One light or shape is at the masthead, and one at each end of the fore yard. These signals indicate that it is dangerous for another vessel to approach within 1000 meters of the mine clearance vessel.
Mine clearance operations are critical for the safety of navigation in coastal waters and shipping lanes. The unique lighting and shapes are visual signals that identify these vessels from afar and indicate that they are engaged in activities which require wide berth due to the potential dangers inherent in their work.
Mariners must be vigilant for such navigation aids and respectful of the operations for the overall marine safety.