Welcome to Unexplained Extra, with me, Richard McLean Smith.

For the weeks in between episodes, we look at the stories that for one reason or other

didn't make it into the show.

In last week's episode, what hides beneath, we looked at the infamous K219 incident and

wondered at the many mysteries of the vast ocean deep.

Some believe a mysterious underwater sound, known as a quacker, may hold the key to discovering

what happened to the ill-fated nuclear submarine.

Thought to be the call of a large, undiscovered sea creature, or even a submersible alien

spacecraft, the quacker is fast becoming a regular fixture in modern maritime folklore.

But you may not be surprised to hear it is only one of many strange sounds to have been

recorded from the depths of the planet's oceans, and there is one sound in particular that

has given rise to some of the most terrifying theories around.

Picked up by hydrophones deep in the Pacific, the sound, known as the blue, may yet prove

the most extraordinary of them all.

As the world slipped from the waking horrors of the Second World War into the mercier dimensions

of the Cold War, the United States Navy were looking for the next opportunity to gain

the upper hand.

As a result, the Committee for Undersea Warfare was established in 1949 with its primary

objective to neutralize any possible underwater threat.

Knowing that sound travels more than four times faster and further in water than it does

in air, the solution was simple.

A monitoring system was created whereby a vast array of underwater microphones dotted

...