During the lockdowns of 2020, global shipping was severely restricted and ocean noise pollution all but ceased. Instead the songs of fish filled the sea.

Crackles, snaps, pops and clicks – that is the noise of a thriving underwater soundscape.

"All of the individual sounds, when they add up, become this orchestra – thousands of different instruments all playing at the same time," says Steve Simpson, a marine biologist at the University of Bristol.

For decades, many people believed the ocean was silent, limited by what our own ears could detect beneath the surface. But in the early 20th Century, when hydrophones – underwater microphones – were introduced to monitor ocean acoustics, we discovered that marine species use a huge variety of sounds.

As human-produced sounds become more prevalent underwater, important animal behaviours such as communication, breeding and feeding may be disrupted. So, since 2010, scientists have been wondering how they could quieten the seas, as a testbed for the impacts of noise on marine species.

Then a pandemic happened – and we were given a rare moment of quiet.

As shipping and commercial boating came to a halt, we could finally hear what the ocean sounded like without the hubbub of manmade noise pollution. Researchers are still studying this year of quiet, investigating the extent to which louder oceans harm marine species.

Peter Tyak, a professor of Marine Mammal Biology at the University of St Andrews, was a founding member of the International Quiet Ocean Experiment (IQOE), a global scientific research programme.

"The core idea of the International Quiet Ocean Experiment was that rather than adding sounds and seeing what happens, maybe you have to go to places and reduce sounds," he says. However, turning the ocean's volume down on a global scale proved to be a costly, logistical nightmare.

But in 2020, Covid-19 put the brakes on shipping and tourism, leading to a 4.1% decrease in global maritime trade. In some economic zones, marine traffic hit lows of 70%, with models estimating a 6% reduction in shipping noise energy worldwide.

This allowed for a natural worldwide experiment of the impacts of sound on marine life.

Across the world, scientists listened to the ocean soundscape before, during and after lockdown, using 200 ocean hydrophones that were already in place around the global ocean.

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When New Zealand entered lockdown on 26 March 2020, boat traffic in the Hauraki Gulf Marine Park – the country's busiest coastal waterway – almost completely stopped. Underwater noise dropped to about one-third of normal levels within 12 hours – allowing the communication ranges of fish and dolphins to increase by up to 65%. For dolphins, that meant their calls could travel around 1 mile (1.5km) further than when hampered by shipping noise.

"Sound is the primary modality for most animals in the ocean," says Miles Parsons, a researcher at the Australian Institute of Marine Science. And the ocean is a noisy place. A recent study suggested that of the 20,000 fish species estimated to be........

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