More than 40 per cent of the world’s population lives within 100km (about 60 miles) of the coast, yet our knowledge of Earth’s oceans is tiny. In some ways, it’s easier to stick humans into space than to send them to the crushing pressures of the planet’s watery depths.
While our understanding of the oceans is improving all the time, we still have plenty to learn… and there are some mysteries that have scientists completely stumped.
1. Why are orcas killing great white sharks?
Something weird has been happening off the coast of South Africa. Since 2017, great white sharks have been washing ashore with their livers ripped out. But who murdered these majestic apex predators?
Reports over the years suggested the grisly findings coincided with increased sightings of a pair of male orcas (otherwise known as killer whales). The pair are known as ‘Port’ and ‘Starboard’ because of their distinctive collapsed dorsal fins (Port’s is collapsed to the left, Starboard’s to the right).
And then, in May 2022, both a drone and a helicopter pilot operating over Mossel Bay witnessed Starboard hunting great whites alongside four other orcas. One of their kills was even captured on camera.
In June 2023, Starboard was then seen operating alone, taking just two minutes to kill a juvenile great white. It is unknown whether or not Port is capable of similar solo feats. Either way, it appears that the duo appear to enjoy snacking on a bit of fishy foie gras.
During a hunt, the orcas flip the unfortunate shark onto its back to induce a trance-like immobile state, before ripping it open and devouring its oil-rich liver.
This behaviour raises questions about how these predation events might impact the area’s intricate ecosystems, as there is evidence that the great whites flee in response to the attacks.
But why did orcas start hunting great whites in the first place? We already know that orcas can be pretty specialist hunters, and in different parts of the world there are populations that prefer to eat fish, seals or sharks.
“In previous years we had orcas coming to South Africa to hunt dolphins. But in 2015 they were documented hunting sevengill sharks for the first time, before being recorded predating on great whites in 2017,” says Leigh de Necker, marine biologist and senior research assistant for Wildtrust’s Shark & Ray Protection Project.
“We don’t know why these shark-hunting specialists are here, and the science can’t keep up with how quickly things are changing. There are theories, but no science to back it up as yet.
"Some people are saying it’s because fisheries are impacting the orcas’ prey source, which is causing them to move. But maybe the orcas have found a hotspot and decided not to leave as there’s an abundance of resources to exploit.”
2. Where did Earth's water come from?
The planet we call home is about 70 per cent ocean, with a surface area of some 360 million kilometres (about 220 million miles) and an average depth of 3,682m (12,080ft), making it the largest habitable area of Earth.
Whichever way you look at it, that’s a lot of water! But the bizarre thing is that while we know that Earth formed around 4.5 billion years ago, we still aren’t completely sure where all that water came from.
Scientists do, however, have some thoughts about how our planet got so soggy.
“Broadly, you can split the theories into two different types,” says Dr Helena Bates, a postdoctoral researcher in meteorite science at the Natural History Museum, London.
“The first thing is that Earth formed in a region of space where there was material like rocks and things like that, but also water in some form.
“The second theory is that Earth formed in a region of space where it was too hot to have any kind of water. And the water that we have on Earth now was delivered post-formation by other bodies in the Solar System.”
While it might seem unbelievable that all of Earth’s water could have splashed down from meteorites and other space rocks, the theory does make sense.
This is because one type of meteorite, the ‘carbonaceous chondrites’, can be made up of 10 per cent water, and early in the life of Solar System there was far more bombarding going on than there is today.
By studying meteorites that land on our planet’s surface, we can gain a better picture of where Earth’s water came from.
“Learning more about comets and asteroids that fall to Earth as meteorites and micrometeorites can tell us a lot about the distribution of water around the Solar System,” says Prof Sara Russell, a professor of planetary sciences at the Natural History Museum.
“The main technique used is to look at the ratio of hydrogen and its heavy isotope, deuterium. In particular, comets tend to have quite high deuterium to hydrogen ratios and so give a distinctive fingerprint.”
According to Bates, most scientists agree that Earth probably got wet due to a combination of things. “I don’t know if we’ll ever work it out,” she says. “But that’s the joy of it!”
3. Who – or what – made the Yonaguni Monument?
Off the coast of Yonaguni, the southernmost of Japan’s Ryukyu Islands, lies a weird rock formation. It was discovered by scuba diver Kihachiro Aratake in 1986, while he was trying to seek out some spots to watch hammerhead sharks.
The unusual monument features perpendicular stacked sandstone blocks, parallel joints, a spiralling ‘staircase’ and strange etchings, leading some people to believe it could be the sunken remains of a civilisation from long ago. Yet no-one knows who made it – if anyone.
One of the people who think it was human-made is Prof Masaaki Kimura, of Ryukyu University in Japan, who led an expedition to ‘Japan’s Atlantis’ shortly after Aratake discovered it.
In 2004, Kimura wrote that “sonic sounding, underwater robot and scuba diving surveys reveal the existence of artificial topographies constructed about 10,000 years ago beneath the sea off the Ryukyu Islands in Japan.
Features were found that look like an ancient city, including stepped pyramids, roads and water canals off the southern coast of Yonaguni Island.”
Other scientists, however, are not convinced. Prof Robert Schoch, a natural scientist at Boston University in the US, made dozens of dives on the site and concluded that the structure is natural.
He pointed to the fact that the rocks in the region break cleanly along vertical and horizontal planes, and similar formations can be seen on the land. He explained away the rock etchings as natural scratches on the sandstone’s surface.
He did concede, however, that there is evidence of ancient civilisations once living on the nearby island.
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4. Where are weird ocean noises coming from?
The ocean is a surprisingly noisy place, with human activity, animal calls, volcanic rumbles and seismic shifts all contributing to the underwater soundscape. But some strange underwater noises have left scientists scratching their heads.
One sound, named ‘the Upsweep’, was first recorded in 1991 by the National Oceanic and Atmospheric Administration (NOAA) and continues to be detected today. It essentially sounds, as its name implies, like a wail steadily rising in pitch.
Each upsweep only lasts a few seconds and appears to radiate from a point between New Zealand and South America. Curiously, activity seems to peak in spring and autumn, but as of yet, scientists have not resolved what’s causing it.
Yet another mystery sound is ‘the Ping’. Emanating from the Fury and Hecla Strait in the Canadian Arctic, the noise was reported in 2016, when hunters in the remote region claimed it was scaring animals away.
The Canadian military sent an aircraft to investigate the sound’s origin, to no avail.
At the time, various theories were circulated, with both mining companies and wildlife activists being accused of making the sounds – all of whom denied involvement. As of yet, we still don’t know where it’s coming from.
But perhaps the most famous noise of all is ‘the Bloop’. One of the loudest underwater sounds ever recorded, the thunderous rumbling was picked up in 1997 via hydrophones placed thousands of kilometres apart across the Pacific.
Over the years, theories started to proliferate as to its origins. Was it from some secret military exercise? Or perhaps it was simply the metallic grindings of boat machinery?
Some were convinced it was the cry of the monster from The Call Of Cthulhu, the 1928 short story by HP Lovecraft. More fuel was thrown on this theory when it was reported that the Bloop’s source was located just 1,500km (930 miles) from where Cthulhu first emerged in the tale.
In 2005, after almost a decade of acoustic surveys and data analysis, scientists finally cracked it – the Bloop was not the wail of a many-tentacled sea creature, but was (slightly disappointingly) the sound of an iceberg breaking away from a glacier.
Of course, climate change means it’s likely that ‘Bloops’ may become more commonplace as the planet’s ice starts to melt at a faster rate.
5. What's going on with the seafloor?
When we think of the sea, most of us think of its lapping waves. But beneath all that wet stuff lies the ocean floor, and that is one deeply mysterious place.
While the entire seabed has been mapped using data collected from satellites, these images only provide a general, sketchy picture of what’s going on down there.
On these maps, whole topographical features such as seamounts are missing, while intriguing relics like shipwrecks also cannot be seen.
However, there are scientists who are working hard to try to decipher this curious place. There is currently a global effort underway, as part of the Seabed 2030 project, to map the seafloor using modern high-resolution technology.
They’ve still got a bit of a way to go – as of June 2024, just 26.1 per cent of the seafloor had been mapped with high-resolution technology.
It’s worth persevering though: according to Seabed 2030, a properly imaged seabed could help boost our knowledge of ocean circulation and climate change, and even assist with tsunami forecasting.
But it’s not only that. An improved understanding of the seabed could help us reshape our theories about Earth’s formation.
This year, researchers from the University of Maryland, in the US, used seismic imaging technology to peer into Earth’s mantle. They discovered evidence of a previously unknown ancient seafloor that may have sunk deep into our planet’s interior approximately 250 million years ago.
“This is just the beginning,” says study leader Dr Jingchuan Wang. “We believe that there are many more ancient structures waiting to be discovered in Earth’s deep interior.
Each one has the potential to reveal many new insights about our planet’s complex past and even lead to a better understanding of other planets beyond ours.”
6. Where has all the plastic gone?
Around eight million tonnes of plastic from land-based sources spew into our oceans each year. While we may know how much of the stuff enters the water, we still don’t know where it ends up.
Research by Prof Kara Law, published in the journal Science in 2015, suggested that between 6,350 and 245,000 tonnes of plastic was floating on the ocean’s surface – a tiny percentage of the total. So where’s the rest of it?
Scientists are keen to figure this out, because if they can get a better idea of where it’s all going, they can gain a deeper understanding of its impact on marine life.
“We know the impact of plastic in the oceans is wide-ranging. The most notorious are the effects of large chunks of plastic – like straws and shopping bags – that impact charismatic animals such as marine mammals, turtles and seabirds.
However, these incidents are the tip of the iceberg. Plastics in the ocean spend very little time in their large, visible form,” says Tom ‘The Blowfish’ Hird, a marine biologist and broadcaster.
“The problem with plastic is that it persists. It doesn’t dissolve. It becomes smaller and smaller. Plastic has now become a microscopic part of seawater, meaning that any animal, large or small, is now living with plastic.”
It appears that no part of the ocean is free from the scourge of plastic. In a 2019 paper published in Royal Society Open Science, British researchers gathered samples of crustaceans called amphipods from six different deep ocean trenches, and found that 65 out of the 90 individuals examined had at least one microplastic particle in their guts.
And it’s in the sea air too – there is evidence that the action of waves on the seashore can aerosolise microplastics into the atmosphere. In fact, one recent study even found microplastics in dolphins’ breath, inhaled when they come up for air.
“The fact of the matter is that we simply do not have the ability to accurately monitor and gather the correct data to give us the true picture of where plastics are going in our ocean and how they’re getting there,” says Hird.
7. Why do some creatures glow?
The ocean is a pretty dark place, but within the inky waters you can see occasional flashes and sparkles of bright light. Bioluminescence, the generation of light by a living organism, has independently evolved around 100 times and is a widespread phenomenon throughout the sea.
In fact, as many as 90 per cent of animals living in the twilight zone – located at depths of 200–1,000m (656–3,280ft) – may create light in some form, which may be used to help find a mate, for camouflage, to lure in prey or to evade predators.
“Bioluminescence can be used in lots of different ways, some of which might surprise us – one of my favourites are the ‘lightsabers’ of lantern sharks, advertising their unappetising spines,” says Prof Jon Copley, a marine ecologist at the University of Southampton.
And bioluminescence has been around for a long time. In 2024, scientists led by Dr Danielle DeLeo from Florida International University, in the US, found that the trait first emerged in a group of marine invertebrates called octocorals around 540 million years ago.
But while we’ve now pinpointed the earliest evidence of its evolution, we still don’t know what sparked animals to emit light in the first place. Copley, however, has a theory.
“It’s important to remember that the ocean prior to 540 million years ago was very different in its conditions compared with today. And in particular, it contained much less oxygen. A rise in oxygen may have contributed to the Cambrian explosion, where a large variety of animals burst onto the evolutionary scene. Life in the shallow oceans would have had to cope with new, more-oxygenated conditions,” he says.
“Bioluminescence is the by-product of an oxidation reaction. So perhaps bioluminescent reactions evolved as part of the development of metabolism for more oxygenated conditions – and then the light by-product from those reactions found a use in signalling and camouflage and so on.”
While bioluminescence is useful for relatively short-range visual communication, it’s worth remembering that it’s not the only sense that’s used in the marine world. “The ocean is a world of smells and sounds as well as bioluminescent lights,” says Copley.
About our experts
Leigh de Necker is a marine biologist and senior research assistant for Wildtrust’s Shark & Ray Protection Project. She has appeared on TV, notably co-hosting Shark Women for Warner Bros Discovery's Shark Week in 2022. She has been published in Nature Ecology & Evolution.
Dr Helena Bates is a postdoctoral researcher in meteorite science at the Natural History Museum. She has been published in the Journal of Geophysical Research: Planets, Meteoritics & Planetary Science and Nature Communications to name a few journals.
Prof Sara Russell is a professor of planetary sciences at the Natural History Museum. You'll find her published in Science, Meteoritics & Planetary Science and Earth and Planetary Science Letters.
Tom 'The Blowfish' Hird is a marine biologist, author and presenter. He has appeared on Shark Week – 6000lb Shark and Ocean Wonders as a TV presenter. Hird is also the author of Blowfish's Oceanopedia.
Prof Jon Copley is a marine ecologist at the University of Southampton. He has been published in Evolution: International Journal of Organic Evolution, Nature Ecology and Evolution and Marine Biology to name a few journals. He is also the author of Deep Sea: 10 Things You Should Know.
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