What is the characteristic of an Isolated Danger Mark?

An Isolated Danger Mark is specifically designed to indicate the presence of a hazard that lies within safe waters. It serves to warn mariners of an underwater obstruction or dangerous area, making it essential for navigation safety.

The mark itself is characterized by being placed above the danger it signifies, ensuring that vessels can see it from a distance, even when navigating in calm waters. This visibility is critical since it alerts the crew of the vessel to avoid this area while still being in otherwise safe waters.

The other characteristics mentioned, such as having a single sphere topmark, indicating safe waters, and having flashing lights, do not accurately describe the attributes of an Isolated Danger Mark. Each of these features belongs to different types of navigation buoys, reflecting varied roles in maritime navigation, thus reinforcing the unique role of Isolated Danger Marks.