"The cable is going underneath here," says Benoit Pirenne, standing at the water's edge on Canada's Vancouver Island. "It's going out 500 miles (800km) in a big loop in the ocean, coming back in the same place."

The Vancouver cable connects a network of scientific instruments on the floor of the north Pacific, some as deep as 1.5 miles (2.5km).

Set up by Pirenne and his colleagues at the University of Victoria, and called Neptune Canada, they continuously monitor the marine environment.

Ocean floor listening posts in the north Pacific off Vancouver Island

The scientists are harvesting large amounts of information, including water pressure readings that help them better understand the movement of tsunamis through oceans, which they hope will lead to more accurate warning systems.

But they are also listening.

Pressure-sensitive microphones pick up the live sounds of everything from whales and shipping to seismic activity and the movement of tectonic plates, and this audio is shared with scientists all over the world.

It's also now available to anyone else with an internet connection.

Ocean 'ears'

"This is like big ears placed on the bottom of the ocean," says Michel Andre, a bioacoustics expert at the Technical University of Catalonia in Barcelona, as he listens to the audio being streamed live from Vancouver.

Many locations marked on listentothedeep.com stream audio in real time

Andre's passion is the study of noise in the ocean, both natural and man-made, and how it affects marine life including whales and dolphins.

"Sound travels five times faster through water than through air, and in the ocean it can travel for thousands of miles," he says.

To help the non-scientific community appreciate just how noisy the oceans have become, he's launched a website called listentothedeep.com which streams the live sounds from Vancouver, and 14 other ocean floor "observatories" around the world.

"Here you can hear the noise coming from a ship," says Andre as he clicks on a green dot in the Mediterranean.

The ship is on the surface of the sea, but its monotonous engine sound eerily dominates the soundscape more than a mile below.

Software developed by Andre's team can identify a range of different sounds.

A graphic representation of the sound of the Japanese earthquake, 11 March 2011

"These coloured bars here represent the sound of sperm whales," he says.

"And if you click here, in the sound library, this is the noise of the Japanese earthquake."

An audio graph of bright orange shows the pattern of the earthquake sound, which came from the Kushiro and Hatsushima observatories off the coast of Japan, in March last year.

On Andre's site, the sound has been sped up to make it audible to the human ear. It's a growing deep rumble, and a chilling experience for anyone who followed the news of the devastating tsunami that struck the Japanese mainland.

National security

For decades, the sounds of the deep sea were considered highly sensitive military data.

During the Cold War, the US Navy set up a network of underwater microphones in many parts of the world to track Soviet submarines.

Now the US Navy says it's not too concerned about friendly civilian scientists listening in, but it is nervous about the opening up of real time ocean sound to the general public.

A spider crab investigates monitoring equipment on the Pacific floor

"It's a region where we have naval operations," says US Navy oceanographer Robert Winokur, referring to the area off Vancouver monitored by the scientists from Neptune Canada.

Continue reading the main story Beneath the surface Leonardo Da Vinci noted in 1490 that sounds carry long distances under water - whale songs can travel thousands of miles

Sound travels about five times faster in water than in air, and is generally faster at greater depths

Marine mammals depend on sound for communication because sight and smell are less effective under water - scientists fear that shipping noise harms whales, porpoises and dolphins

Fish also hear sounds within a limited frequency range, and produce sound with their swim bladders and teeth

Allowing everyone to listen to that part of the ocean poses a threat to US national security, he says, because someone - he won't say who - might use that information to work out the type and location of US naval vessels.

To reduce that threat, the US and Canadian navies have struck a deal with the scientists that effectively allows them to censor the sound of naval operations.

"Rather than it being sent to the internet in real time, we divert it to a naval facility where the data are screened, and then returned as soon as possible to the University of Victoria," says Winokur.

In other words, the sound is scrubbed to remove the noise of naval vessels.

Widening the net

The US Navy says it hopes to work out similar arrangements with other ocean monitoring projects elsewhere in the world.

But Cornell University ocean acoustics expert Christopher Clark doubts it will succeed.

"The strategy they're using with Neptune (the Vancouver project) is a dead end," he says.

Clark insists the US Navy doesn't own the ocean acoustic environment and should accept that what was once military technology is now in the hands of civilians.

A rattail fish swims past monitoring equipment in the Pacific

"The cat's out of the bag, the horses are out of the barn, whatever the metaphor is, it's happening," he says.

The number of observatories on Michel Andre's website is growing. The site currently links to listening networks only in the northern hemisphere.

Soon 11 more will be added, connecting to microphones in the South Pacific, South Atlantic and the southern Indian Ocean.

That will open up an even larger swathe of the ocean deep to our ears.

Listen to more on this story at PRI's The World, a co-production of the BBC World Service, Public Radio International, and WGBH in Boston.

Picture credits: SPL, Reuters, Neptune Canada