Image: Sebnem Coskun/Anadolu Agency via Getty Images

Compared to life on the surface, scientists know extraordinarily little about the deepest depths of the sea. In fact, we’ve actually explored and mapped less than 20 percent of the world’s oceans, despite the fact they cover more than two-thirds of the planet. As researchers continue to traverse new depths, then, they’re constantly making mind-blowing discoveries – from long-lost cities to incredible new species.

Image: YouTube/University of Nottingham

20. Artifacts from an ancient naval battle

It’s not every day you dig up relics of a millennia-old war – but that’s exactly what a group of British archaeologists did in 2013. The astonishing artifacts date back to a climactic confrontation between the Romans and Carthaginians, at the culmination of the First Punic War in 241 B.C. The Battle of the Egadi Islands ended in victory for the Romans, and set the Mediterranean power on course to establish its empire.

Image: YouTube/University of Nottingham

The ancient battle site, which is located just off the Sicilian coast, turned up a veritable smorgasbord of incredible finds. Among them were two beautifully-preserved Roman military helmets, multiple warship-mounted battering rams, and a fascinating iron sword, which measures nearly 28 inches long and 2 inches wide. The relics have since been taken to the U.K.’s University of Nottingham for analysis and restoration.

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Image: Giovanni Dall’Orto

19. Ancient Roman medicine

In the late 20th century, a group of archaeologists began excavating the Relitto del Pozzino – a shipping vessel that had sunk off the Tuscan coast some 2,000 years previously. The cargo included some significant discoveries, chief among which was a set of cylindrical tin containers. That’s because despite sitting on the ocean floor for two millennia, the tins were still sealed.

Image: Sailko/CC BY 3.0

When they opened the tins, the archaeologists found six circular pills, which have since offered an intriguing glimpse into ancient Roman medicine. In 2013 a group of scientists published the results of a chemical analysis of the tablets, which determined that they contained zinc. Owing to the ingredient’s anti-inflammatory properties and the etymology of the Latin word for the medicine – collyrium – it’s believed they may have been used to treat eye conditions. The term derives from a Greek phrase meaning tiny, round loaves which perfectly describes their form.

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18. An ancient Croatian shipwreck

In 2008 a group of fishermen working off the Croatian coast spotted the remains of a boat lurking beneath the surface. And at the time, they simply assumed it was a modern wreckage. But when early investigations dated the craft to pre-Roman times, a team of archaeologists decided to more closely analyze the archaic vessel.

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Image: W.Strickling/CC BY-SA 4.0

The researchers finally published their results in 2014 – and revealed that the boat actually dated back some 3,200 years, to the 12th century B.C. The incredible discovery was what’s known as a “sewn boat,” a type of wooden vessel that’s literally stitched together with ropes. The technique was common even after the invention of metal clasps and rivets, and was often used to reduce shipbuilding costs before the Roman era.

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17. Swedish Stone Age artifacts

It wasn’t quite the “Swedish Atlantis” that media sites trumpeted it to be at the time, but the 2014 discovery of ancient Stone Age relics in the Baltic Sea was still hugely exciting. After all, the artifacts dated back some 11,000 years – and revealed much about life for Stone Age Swedes.

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Among the astonishingly well-preserved items were tools, ropes, and even the bones of a long-extinct animal. The objects survived over the millennia through a crucial combination of environmental factors: namely, limited oxygen and organic-rich sediment. But archaeologists quickly ruled out any hopes of finding a “Swedish Atlantis,” because the Stone Age Swedes were strictly nomadic.

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16. Underwater ruins near Japan

In 1995 a diver swam out from the Okinawa shore – and stumbled upon an absolutely mind-blowing deep-sea discovery. Lying dormant beneath the water’s surface was an amazing arrangement of seemingly man-made structures. Archaeologists, divers and media outlets flocked to the fabulous find, but nobody could figure out its origins.

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Image: Melkov/CC0 1.0

Expert divers eventually mapped out the entire location, and realized that the region resembled what appeared to be a lost city. Enormous stone arches, apparently paved streets, and features resembling stairways, pylons and plazas all gave credence to the theory that this was a sunken settlement. However, other scientists contend to this day that the formations are simply a natural consequence of tectonic movement.

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Image: University of Athens via The Telegraph

15. Natural underwater ruins

If you dive beneath the water near the Greek island of Zakynthos, you’ll eventually come across a mysterious collection of stone relics that hint at a lost city. However, the reality is even more astounding. While the debris resembles ancient Greek architecture, it actually formed completely naturally more than 5 million years ago.

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Image: University of Athens via The Telegraph

The peculiar phenomenon occurred through a process known as “concretion,” in which a bacterial byproduct reacts with methane. The result is a type of naturally-occurring cement, which helped to shape the stone into its familiar formations. Alongside a number of notable columns, the microbes are also responsible for what looks like a paved floor, deceiving the snorkelers who originally stumbled on the ruins.

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Image: YouTube/LundUniversity

14. A sunken Egyptian city

In the 2nd century B.C., a tremor on the threshold of ancient Egypt brought the fabled port city of Thonis-Heracleion crashing into the waters below. And it laid there for more than 2,000 years – until a group of divers discovered a rock fragment that once belonged to a statue of Hapy, the god of fertility. Emboldened by their find, the divers continued to search the area. And they’d soon turned up countless relics of a long-forgotten land.

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Among the ruins were coins, jewelry, busts, and remnants of pottery and temples. British Museum curator Aurélia Masson-Berghoff told The Guardian newspaper in 2016, “As an archaeologist, discovering a tomb is exciting, but it’s the tomb of one individual. Discovering a whole city, which was home to thousands and thousands of people over more than a thousand years… Well, that’s something else.”

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13. A plastic-polluted crustacean

It’s not just underwater ruins that lie in the deepest depths of the ocean, however. Yes, the most unknown regions of our planet also house all kinds of weird and wonderful wildlife. But even in those cold, dark, and mostly inhospitable environments, where scientists have only ventured in purpose-built submarines, humanity’s impact has already been felt.

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Indeed, in March 2020 a group of British researchers unveiled a new species of crustacean, which was caught at a depth of some 20,000 feet inside the Marianas Trench, famed as the deepest place on Earth. And they’d bestowed it with the name Eurythenes plasticus – a reference to the plastic particles they’d found in its innards. In doing so, the scientists were apparently aiming to draw attention to the severity of our environmental impact on the oceans.

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Image: NOAA Office of Ocean Exploration and Research

12. Red coffinfish

The eye-catching red coffinfish is notable not just for its distinctive color, but the way it’s evolved to survive in its low-resource environment. That’s because this mysterious creature can actually hold its breath for up to four minutes, allowing it to conserve energy – and even see off predators. Indeed, by taking in more water the fish can inflate its normal body size by an additional 30 percent.

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Image: NOAA Office of Ocean Exploration and Research

That’s not the only evolutionary advantage the coffinfish – or “sea toad” – has developed over the millennia. In fact, it also has fins that allow it to “walk” on the seabed, reducing its need to expend energy swimming. Its diet, meanwhile, is similarly low-effort: it simply opens its mouth and eats whatever floats by. But given that prey is scarce in the deep ocean, these traits are fundamental to the coffinfish’s continued survival.

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Image: via Wikimedia Commons

11. Luminescent brittle star

There are thousands of different species of brittle stars in the ocean, with the majority found in particularly deep waters. As their name suggests, they belong to the same family as the starfish, and use their five spindly arms to navigate the ocean floor. But a small number of these species boast an additional trait: bioluminescence.

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Image: Hans Hillewaert/CC BY-SA 4.0

The brittle star’s innate ability to illuminate its arms is primarily a defense mechanism, used to scare away predators. However, scientists suspect the sea creature’s bioluminescence may also activate for other reasons. For instance, it’s thought that the fascinating ability may be used in mating, and can also be indirectly triggered by environmental factors, such as the quantity of metallic particles in the water.

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Image: Alaska Fisheries Science Center, NOAA Fisheries

10. Blobfish

The Psychrolutes marcidus stormed to internet infamy in 2013 when it was voted the ugliest animal in the world. The rarely-seen creature, which typically resides up to 3,900 feet beneath the ocean, was first caught off the New Zealand coast in 2003, and has since become iconic for its unflattering features. However, the blobfish’s unfortunate moniker may not actually be that accurate.

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Image: Alaska Fisheries Science Center, NOAA Fisheries

You see, the Psychrolutes marcidus has spent millions of years evolving to cope with the realities of deep-sea life. For one, their soft bones allow them to withstand the otherwise-crushing pressure experienced at the bottom of the ocean. But when fishermen drag them to the surface, that lack of internal support causes them to collapse into the blob-like form for which they’re known.

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Image: Ahmed Abdul Rahman for MDC Seamarc Maldives/CC BY-SA 4.0

9. Corallimorpharia

This outlandish organism is related to plenty of similarly-tentacled animals, including jellyfish, anemones and hard corals. Unlike their fishy family members, though, corallimorphs don’t have exoskeletons. While that separates them from other coral creatures, they’re still popular among aquarium hobbyists for their resilience and color.

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Image: Nhobgood Nick Hobgood/CC BY-SA 3.0

Corallimorphs can be spotted in a variety of marine habitats, in waters both shallow and deep, from the tropics to the poles. The tropical species are undoubtedly the most visually arresting, though, with dozens of short tentacles protruding from their flat, disc-like bodies. And those that live in the deepest depths of the ocean can grow to stupendous sizes of up to a meter in diameter.

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8. The faceless fish

The “faceless fish” was first spotted all the way back in 1873 by the crew of HMS Challenger, during the first ever global oceanographic expedition. Yet that was the fish’s last recorded appearance in Australian waters for more than a century; in 2017, however, a group of scientists trawling an enormous abyss managed to pull one up. The ultra-rare sea creature was found floating almost two-and-a-half miles below the surface.

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Image: YouTube/Qldaah

Given its obscurity, the crew were initially perplexed by the fish and only determined its origins by looking it up in journals aboard the trawler. They then discovered its unusual nickname, earned from the curious position of its mouth, which is located on the underside of the animal. As a result, looking at it in profile view gives the impression that it’s actually completely faceless.

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Image: NOAA Ocean Exploration & Research/CC BY-SA 2.0

7. Dumbo octopus

Just like the classic Disney character, the Dumbo octopus uses its enormous ears to traverse its environment. Rather than flying through the skies, though, the unusual animal must navigate the deep ocean – down to at least 13,000 feet, and possibly deeper still. That makes it the deepest-living octopus known to man, but that’s not the only reason the 15 species of Dumbo octopuses are notable.

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Image: NOAA Ocean Exploration & Research/CC BY-SA 2.0

For one, Dumbo octopuses don’t have ink sacs, because they live deep enough that they rarely come across predators. And as they’re also pretty rare themselves, they’ve evolved to make the mating process as productive as possible. Indeed, female Dumbo octopuses are essentially prepared to reproduce at all times, and can even induce conception long after the initial mating process is complete.

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Image: Gerringer M. E., Linley T. D., Jamieson A. J., Goetze E., Drazen J. C./CC BY 3.0

6. Mariana snailfish

The deepest depths of the ocean are home to many species that we’re still discovering to this day. Take the Mariana snailfish, for instance, which was found by a group of researchers from across the globe in 2017. At the time, it was the deepest fish ever to be picked up from the seabed.

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Image: Gerringer M. E., Linley T. D., Jamieson A. J., Goetze E., Drazen J. C./CC BY 3.0

As its name suggests, the Mariana snailfish was collected from the Marianas Trench – the deepest area of ocean on Earth. It accordingly thrives at incredible depths of about 26,000 feet, with a handful of the species spotted as deep as 26,830 feet. Having evolved to withstand the intense pressure of the ocean floor, the snailfish has found a home where predators are absent and food is plentiful.

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Image: Norman Kuring, GSFC Ocean Color Team

5. Hundreds of huge craters

Just north of Norway, in the Barents Sea, there’s a vast expanse of hundreds of huge, underwater craters, stretching out up to half a mile wide each. They’re not some kind of deep-sea equivalent of crop circles, though. Indeed, they actually formed entirely naturally some 12,000 years ago, towards the end of the Ice Age.

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Image: Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC

When methane is created in the depths of the Earth, it travels to the surface and is usually released straight into the atmosphere. But 15,000 years ago, the Barents Sea was merely an ice sheet, and the methane that formed below it had nowhere to go – so it froze into methane hydrate. When the ice melted, the methane converted back into a gas, creating massive bulges in the seafloor. Eventually, many of them exploded under the pressure, forming the giant craters you can see today.

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Image: YouTube/Monterey Bay Aquarium Research Institute (MBARI)

4. A squid graveyard

In 2015 a team of researchers stumbled upon a strange sight on the Gulf of California seabed: dozens of dead squid. The carcasses were spotted over a series of expeditions at depths of at least 3,300 feet. And according to lead biologist Henk-Jan Hoving, they’ll have represented a rare bonus for other fish in the deep ocean, where food is relatively scarce.

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Image: YouTube/Monterey Bay Aquarium Research Institute (MBARI)

“You have to get very lucky to see [a squid carcass],” he told Monterey Bay Aquarium Research Institute in 2018. “They likely get consumed within 24 hours.” The scientists believe the squid were females, which deplete their energy resources during the reproductive cycle and die once their eggs are released.

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3. The “ice finger of death”

While scientists have known about brinicles – literally an icicle of brine – since the mid-20th century, very few had actually watched them form. Until 2015, that is, when one of the frozen marvels was captured forming on camera. The process is triggered by the cracking of polar ice on the sea’s surface, which results in brine leaking out into the ocean.

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Image: YouTube/GodsLucCk

Since the highly saline brine weighs more than the ocean water, it spools down to the seabed. And because it’s so exceptionally cold, it also freezes the water around it – forming the icicle shape from which it takes its name. Its more sinister nickname, meanwhile, comes from its tendency to prove fatal to nearby life on the ocean floor, including starfish and sea urchins.

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Image: Awashima Marine Park/Getty Images

2. Frilled sharks

The lesser-spotted frilled shark looks like something out of the prehistoric era, but it’s very much alive today. Chances are, though, you’ll never see one in person, because they’re known to reside deep in the ocean. In fact, the frightening shark has been seen at depths of up to 5,150 feet.

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Image: Awashima Marine Park/Getty Images

The frilled shark, which takes its name from the appearance of its fringed gills, was first recorded in the late 1800s. But thanks to its relative rarity, little has been learnt about the species in the century since. What scientists do know is that the frilled shark swims much like an eel, and can swallow prey whole thanks to its extraordinarily flexible jaw.

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Image: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team

1. An undersea river

In 2010 a team of researchers from the U.K.’s University of Leeds sent a robotic submarine below the Bosphorus strait through Istanbul. And what they found there confirmed a long-held scientific theory: rivers can and have formed far beneath the surface of the oceans. Indeed, one of the waterway’s two extremely strong currents has carved out an enormous undersea river, which carries important nutrients out to the Black Sea.

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Image: YouTube/AB TV

Over the years, sonar scans have turned up a multitude of deep-sea channels, some of which reach several miles in width. However, none of these channels have had water flowing through them. The Bosphorus river, then – which is technically the sixth-largest in the world – is a unique find, and proves that the other channels were indeed formed by vast undersea rivers.

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