The military’s active sonar works in much the same manner as echolocation – the communications system that scientists believe is used by bats, dolphins and many species of whales. A sound wave of a particular frequency is sent out into open water. When it hits a large object, such as a whale or a silent enemy submarine, it bounces back and is measured by instruments that can plot the object’s size, shape and location.

Today’s nuclear and air-propulsion submarines are notoriously difficult to detect using passive sonar, which has been used for decades to locate the enemy by listening for the grinding of their diesel engines.

So the U.S. Navy has spent the past 20 years developing active sonar at a wide range of frequencies, both stationary systems that operate off buoys or wharves and Surveillance Towed Array Sensor Systems, which are suspended behind ships. But the most promising Navy innovation – and the most controversial – is known as SURTASS Low Frequency Active Sonar.

The LFAS system consists of a 35-ton block of 18 huge, underwater speakers and dozens of microphones. The speakers emit a consistent low-frequency tone between 100 and 500 hertz, which travels out into the water at a depth of several hundred meters. The low frequency permits the sound to travel tremendous distances, detecting objects many hundreds of miles away.