The GIUK gap is a naval warfare bottleneck in the northern Atlantic Ocean between Greenland (G), Iceland (I), and the United Kingdom (UK). Such geographic “chokepoints” are of strategic importance, as they form relatively narrow passages that are more easily patrolled and secured. During the Cold War, the West kept a close eye on Soviet submarine activity through the gap, while the Soviets monitored it for any forces headed their way. What monitoring system did the US develop for this purpose? Discuss
Source: The Free Dictionary