What is a common top mark for an isolated danger buoy?

An isolated danger buoy is designed to indicate a significant hazard or an obstruction that is surrounded by navigable water but isolated from other dangers. The common top mark for an isolated danger buoy is two black spheres stacked one above the other. This specific marking helps mariners easily identify the buoy and understand that they should navigate with caution in the vicinity, as it denotes a location that may contain underwater hazards.

The use of two black spheres is a standardized marking that provides a clear visual signal that differentiates it from other buoy types, aiding in safe navigation. This consistency is essential for ensuring that all vessels can recognize and respond appropriately to isolated dangers in maritime environments.