It's a geological feature so massive, so vast and so imposing that it makes Mount Everest look like a mole hill by comparison. Unlike Everest, though, it's nearly invisible and will be forever unseen by the unaided human eye. It's the Mariana Trench, an underwater gash in Earth's crust that's five times longer than the Grand Canyon and much, much deeper.
In fact, the Mariana Trench is the deepest part anywhere in the Earth's oceans. Estimates vary a little, but at its blackest depths, a crease called the Challenger Deep, this abyss is close to 36,037 feet (10,984 meters), or about 6.8 miles (10.9 kilometers) deep. If you inverted Everest and plunged it into the Mariana, it highest craggy peak would fall short of the bottom by more than 7,000 feet (2,134 meters).
The trench forms where two tectonic plates (jigsaw-puzzle-shaped pieces of Earth's crust) crunch into each other. As the plates collide in slow motion, the edges push downward into a V shape, creating a valley that has no equal on our planet.
It's a place so foreign that until recent decades, scientists had almost no clue as to what — if any — sort of lifeforms might be hovering there. If you plunge deeper than 3,280 feet (1,000 meters) into the ocean, there's no sunlight to spawn life. Water temperatures often settle in at just above freezing. And food isn't particularly plentiful.
The water pressure in the trench is nearly 1,000 times greater than at sea level. The pressure is so high that it will crush nearly any creature (or manmade object), unless that animal or vessel is built specifically to withstand those extremes. This particular area of the sea, then, is more than a little inhospitable. But these expanses are not lifeless.
A few manned and unmanned vehicles have parted the waters of the trench in recent years, proving that there are indeed organisms living and even thriving in this nearly alien environment. Fittingly, some of these critters are wonderfully strange.
Let's shine a dim ray of sunshine through this watery, mysterious underworld and peek at a few of the most bizarre living things on the planet. But these species aren't just weird; they're also some of toughest animals around.
It's an octopus that Walt Disney would've invented for one of his animated films. It's the dumbo octopus, which has cute little Dumbo-the-Elephant-like ears atop its 12-inch (30-centimeter) body. This adorable animal also has precious wiggly eyes and a delightful puckered mouth that only add to its cartoonish looks.
This octopus may look dainty, but it's actually durable enough to make it the deepest dwelling octopus known to science. It prefers to make its home all the way down between 9,800 and 13,000 feet (2,987 and 3,962 meters).
When you think of octopuses, you probably envision a bulbous mantle sprouting eight dangly tentacles. The dumbo, however, falls into a category of so-called umbrella octopuses with webbed tentacles that give them, well, an umbrella appearance. The effect is something like a starfish with a balloon head emerging from the center.
Unlike most octopuses, this species doesn't chomp and grind food with a beaklike mouth. Instead, it simply swallows its prey whole. So if you happen to be on the dumbo octopus's menu, it probably doesn't seem nearly as cute.
If the dumbo octopus is one of the most harmless-looking ocean animals, then the deep-sea dragonfish is the opposite. With oversized teeth and a hideous face, the dragonfish is an assassin of the unfathomable deep. Although it's a fish, it has no scales, but instead a slippery, slimy skin that resembles an eel's.
Dragonfish, which are about 6 inches (15 centimeters) long, prefer to swim between 700 and 6,000 feet (213 and 1,828 meters) under the surface, where the waters are lightless and cold. Like many deep-water creatures, this species relies heavily on bioluminescent body parts, which leverage internal chemical reactions to produce an eerie glow.
The fish may use this glow to communicate with other fish or to provide camouflage. It also dangles a lighted barbel, or whiskerlike protrusion, from its lower jaw. Other fish are attracted to the barbel, mistaking it for an easy meal. But in a flash, the dragonfish gets lunch instead.
Some dragonfish have also evolved the ability to produce a red glow — an unusual color of light for ocean dwellers. They may use their reddish hue to signal their brethren, but it's more likely that they're using the red lamp to illuminate prey just before launching an attack.
Light is a rare and precious thing in the midnight zone of the ocean. The ability to detect even a glimmer of sunshine can mean the difference between catching a meal and being one. So creatures of the trench, like the barreleye fish, evolve unusual features to use shreds of light to their advantage.
How unusual? Well, for starters, this fish has a transparent head. Inside that head are two sensitive barrel-shaped eyes which are most frequently pointed upwards, allowing the fish to see silhouettes of its prey. As for the clear head, scientists think this feature may simply allow the fish to collect just a little more light, which may give this strange animal a bit more of an advantage over its competition.
The barreleye fish wasn't even known to humans until 1939, when it was pulled from its habitat 2,500 feet (762 meters) below the surface. Even then, the specimens were less than ideal because they collapsed in the pressure changes from deep to shallow.
Now that researchers have access to deep-diving remotely operated vehicles (ROVs) equipped with lights and cameras that can withstand the pressure, they're able to observe the barreleye more closely. Yet this odd fish still holds many secrets, leaving scientists puzzling over its lifecycle and reproduction patterns.
Jellyfish are fairly common sea creatures, washing up on shores and clogging fishermen's nets. The benthocodon, though, is an unusual type of jellyfish that prefers an environment far out at sea at depths of more than 2,500 feet (762 meters), often right on the seafloor.
These are compact jellyfish with a rounded top, called the bell. The bell is typically smaller than three-quarters of an inch to 1.2 inches (2 to 3 centimeters) in diameter, and it's laced with an estimated 1,500 wispy red tentacles, which it uses to whisk itself through the water. The benthocodon dines on small crustaceans and foraminiferans, tiny unicellular organisms.
Although many types of jellyfish are transparent, the benthocodon has an opaque reddish coloring on its bell. Scientists believe that this hue may help mask the bioluminescent glow of the tiny animals that the jellyfish eats, hiding the benthocodon from danger.
Like so many animals in the trench, this species remains a mystery to scientists.
If a fish has the word "devil" in its name, it's a safe bet that it's going to be freaky. The seadevil anglerfish does not disappoint — it features a whole list of fascinatingly strange characteristics.
It's hard not to start with the seadevil's looks. As its name strongly hints, this is a fish that could've swum up straight from hell, with its misshapen body, razorlike teeth and cold death stare. Although they're bizarre and scary looking, at least they're not huge. Females generally top out at 8 inches (20 centimeters) long. The males are much smaller, at maybe an inch (2.5 centimeters) long.
In a strange evolutionary twist of reproduction, the males actually fuse themselves to the females. Their fins, teeth and eyes disappear, along with a few internal organs, ultimately turning the two individuals into one. What's left of the male's body essentially becomes a storage tank for sperm that will help fertilize the female's eggs when the time is right.
As an anglerfish, the seadevil doesn't dart after it prey. Instead, it has a protrusion from its forehead that dangles a glowing lure to attract starstruck, luckless animals. With its huge, gaping jaws, the seadevil can actually devour creatures larger than itself.
If you've ever seen the iconic movie "Aliens," you've been haunted by dreams of toothy, bald creatures bursting from your chest and snapping at your face. Now picture a shark with just such a face swimming around in the darkest, deepest seas. That's the goblin shark, an aptly named monstrosity from your worst nightmares.
Goblin sharks have a protruding snout that looks like a pointy sword. Just below the snout are a set of protruding jaws that appear to be mismatched for the shark's face, as if evolution spun the wheel of ugly and the goblin shark lost in the worst possible way. What's more, these sharks aren't your stereotypical gray color. Instead, their skin has a distinct pink hue.
If you're ever in the water when a goblin shark passes by, you'll find yourself dwarfed in size — they can grow as big as 18 feet (5.5 meters) in length. Fortunately, you're unlikely to encounter such a beast. These sharks typically cruise way down to 3,000 feet (914 meters), and the older they get, the deeper they dive.
As with a lot of deep-sea animals, science knows very little about goblin sharks. No one knows exactly how they reproduce, and a pregnant female has never been captured. So like the goblins of fairy tales, these fish remain a mysterious and fantastic example of just how diverse life on Earth can be.
There are a lot of odd-looking fish in the sea, but not many of them resemble humans' hand tools. The deep-sea hatchetfish resembles a silvery swimming hatchet.
There are more than 40 species of hatchetfish. All of them have ridiculously skinny bodies, and many of them have shiny scales, too, which adds to the metallic, hatchetlike appearance. They're small fish, and even the biggest types grow only to about 6 inches (15 centimeters) long. Their delicate looks belie serious ruggedness, because these fish are found in depths pushing nearly 5,000 feet (1,524 meters).
Hatchetfish have bioluminescent bodies, and they can alter the brightness of their glow depending on how much light is filtering from above. In doing so, they're counterilluminating their bodies in a clever camouflage technique. Their dim, self-produced light reduces their silhouettes, making it much more difficult for predators to spot them from below.
Frilled sharks look like a mix-and-match special from the discount aisle at your local evolution convenience store. They have the rounded body of an eel paired with a flattened head that would like right at home atop a terrestrial dinosaur. Perhaps that's fitting, because like many sharks, this species has ancient roots that extend back nearly 80 million years.
The shark derives its name from six rows of frilly gills that grace its body, which grows up to 6 feet (1.8 meters) long. Just as notably, the shark wields more than 20 rows of wicked, trident-shaped teeth that will tear into any bit of flesh that passes near them.
Frilled sharks probably spend most of their lives near the ocean's bottom, and they like waters more than 4,000 feet (1,219 meters) deep. On the rare occasions that people snag them and bring them to the surface, the sharks almost always perish immediately, making it very difficult for us to observe their behavior and lifecycles.
For years, many people assumed that frilled sharks swam and hunted like eels. Some researchers think an awkward arrangement of internal organs would make that kind of movement impossible. Instead, they say, these sharks may actually strike their prey with the action of a land-based snake, making them even weirder.
Like wraiths of the abyss, telescope octopuses float and dangle in the deepest currents of Earth's oceans. Unlike most octopuses, this one doesn't flit about on the sea floor. Instead, it drifts through the water column at depths greater than 6,500 feet (1,981 meters), and it doesn't swim horizontally, but rather suspends itself vertically, perhaps to make it harder for deeper predators to see its shape.
If you were lucky enough to spot a telescope octopus, you'd probably wonder if the underwater pressure was making you see things. Its body is so clear that it's nearly transparent, and between each of its eight tentacles is a delicate webbing that lends this species a ghostly shape.
In that cellophanelike flesh, you'll see two protruding eyeballs unlike those found in other octopuses. These eyes provide wider peripheral vision so that the octopus can see predators and prey alike. Like something out of a sci-fi movie, those eyes also rotate, perhaps offering the creature an even better way to see through the darkness of its deep haven.
Officially, it's called the osedax, and its name, as well as its feathery appearance, make it seem like a plant from a Dr. Seuss book. But this worm also goes by fiercer monikers such as bone worm or zombie worm, and it can consume the rock-hard bones of some of Earth's biggest animals, including whales.
The zombie worm secretes acids to help it access the inner contents of those dead whale bones. Then, it uses symbiotic bacteria to convert the bone's proteins and fats into nutrients that serve as its food. Its feathery "branches" wiggle in the water, pulling in oxygen to keep the worm alive.
Female zombie worms can grow up to around 2 inches (5 centimeters) long. The males are microscopic by comparison, and females will collect a male harem of these tiny guys on their bodies. Eventually, the males find their way into the female's oviducts. The female releases her fertilized eggs into the water, the worm's lifecycle begins anew, and the zombie worms go about their business of cleaning up whale debris in the ocean's darkest corners.
Thanks to better technologies, we humans have finally begun to peer into the blackness of the Mariana Trench. Still, this underwater canyon is one of the most unexplored places on our planet, and it will likely remain so until we find new ways to peer into the depths without risking being crushed or drowned (or breaking our research budgets).
So like the trench itself, the animals that live there will continue to be mysteries, too. They may be our Earth cousins, but considering how little we know about them, they might as well be from another world.
HowStuffWorks shines a light into the deep ocean to look at the Dumbo octopus.
Author's Note: 10 Weird Creatures From the Mariana Trench
More than two decades ago, I was fascinated by "The Undersea World of Jacques Cousteau," a documentary-style TV show that explored the world's oceans. The crew poked their cameras into every underwater nook and cranny they could find, showing millions of viewers a new perspective on life beneath the waves. Although these days our cameras and scientific technology have improved immensely, we still have more questions than answers about life in the deepest parts of our seas — a testament to just how difficult it is for us to go adventuring in some parts of our own planet.
More Great Links
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