source: United States Navy
This map displays SOSUS Atlantic listening stations lining the continental shelf of the United States.
Their mission was to detect and track submarines in the Atlantic ocean.
This was accomplished through a distributed underwater network of angle sensitive audio sensors. These directional microphones are connected at the end of a many miles long submarine cable out to the continental shelf that drops thousands of feet to the ocean floor. (The white coloring in the relief map reveals the shelf face feature.)
The SOSUS listening stations are located on coastal land. Their underwater hydrophone sensors are many miles out at sea (80 miles or more), and often half a mile deep in the ocean’s bottom or on the edge of the shelf (or island’s seamount in the case of Bermuda).