"The big smell, fish everywhere, whales busting up through it." Simon Millar was sailing off the coast of Australia, when something extraordinary happened. "It was incredible. They were just everywhere."

Over 100 humpback whalesOne of thirteen species of giant whale, the humpback can be found in every ocean., each as big as a bus and twice as heavy, breachingWhen a whale comes to the surface. Humpbacks are especially popular with whale watchers for their acrobatic ariel displays. the ocean's surface to feed. This spectacular sight has only once before been caught on film in Australia's waters.

Polar biologist Jen Jackson says these megapods are "a positive sign" that whales are recovering from near extinction, only 60 years ago. 

Counting whales is not easy. They migrate huge distances and can hold their breath for over an hour. But scientists estimate there are now around 80,000 humpbacks worldwide. Australia is even considering removing them from their list of threatened animals.

It's a remarkable turnaround. For centuries, they were hunted for their meat, fat and bones. Whalers sold baleen to weave baskets and make corsets. Blubber was boiled to extract oil for soap, margarine and lamp fuel.

In the last 1,000 years, according to one study, humans have killed tens of millions of whales and reduced the global population by between 66 and 90%.

It's a tragedy for both the whales and the planet, says Vicki James at the charity Whale and Dolphin Conservation. Marine biologist Asha de Vos compares it to a giant "game of Jenga". Removing the world's biggest animal from the oceans has caused entire ecosystems to collapse.

Many of those are hidden on the seafloor. When a dead whale sinks to the ocean bed it becomes a whale fallThese unique habitats exist 1,000 metres below the surface and were first discovered in the 1980s with deep-sea sonar and submersibles., a habitat and source of food for as many as 200 different species, from hagfish to sea snails.

A whale fall also buries two tonnes of carbon that would otherwise be released into the air. This is a natural form of carbon sequestrationCapturing and removing carbon from the atmosphere. Artificial methods include adding iron to the oceans to stimulate algal blooms and mechanical trees that suck CO2 from the air. and could help cool the Earth by reducing the amount of CO2 in the atmosphere.

But according to recent research by marine scientist Andrew Pershing, whales have an even bigger weapon against climate change. Their poo is a rich fertiliser that supports the ocean's forests of algae. This phytoplankton captures 40% of all CO2 produced, equivalent to four Amazon rainforests.

Pershing's research shows a fully-restored whale population could help save the planet. But will numbers continue to recover?

In the 1970s, a successful campaign to "save the whales" led to a global moratorium on commercial whaling. Since then, whale tourismWhale watching is a growing industry. There is an ongoing debate about how much stress boats put on whales. Iceland recently piloted the world's first "silent" electric-powered boat to watch whales. has become more profitable than hunting. But according to marine scientist Olaf Meynecke, whales face "new threats" from plastic waste, ship strikes and overfishing.

And scientists warn the biggest problem may be noise pollution. Toothed whales find their food using echolocation. Humpbacks communicate with long complex songs that travel tens of kilometres and can last up to 22 hours.

Shipping and air traffic create sounds that interfere with this behaviour. One study found stress levels dropped in whales shortly after the 2001 terrorist attacks, as traffic was temporarily suspended. Covid-19 has had a similar impact on human activity, with scientists dubbing 2020, the Year of the Quiet OceanThe International Quiet Ocean Experiment found that the drop in noise pollution increased the range of dolphin communication by up to 65%.

Yes: We saved the whales. Now they can save us. Oceans cover 71% of the planet and are home to 94% of life. Whales are a keystone species in this ecosystem. They could play a major role in rebuilding our natural world.

No: Our hunting has driven these animals to the brink of extinction. They can never recover in the time needed to prevent irreversible damage. In that time, climate change will have warmed the oceans and changed currents, threatening the survival of the world's largest animal.

Or... Whales are just part of an ecosystem that, when it works, supports itself and other life. We must not rely on whales to save us from the damage we are creating - instead, we must work with them to rebuild the planet.