10 Strange Discoveries About the Deep Ocean

The world’s oceans stretch across more than 360 million square kilometers. The Pacific is the largest at around 165 million km², followed by the Atlantic at about 85 million km². The Indian, Southern, and Arctic oceans round out the list. Beneath the surface, the deep ocean extends into darkness, extreme pressure, and extraordinary depth. It covers 71% of Earth’s surface and contains vast majority of the planet’s habitable volume. The deepest known point is the Challenger Deep in the Mariana Trench, which reaches around 36,000 feet.

For decades, the deep ocean has been both celebrated and feared. Its alien beauty draws scientists while unsettling those wary of what lies in the dark. Groundbreaking discoveries, including early missions such as Alvin, have pushed exploration into extremely deep trenches. Aside from shipwrecks and debris, explorers continue to find strange things in the ocean’s darkness. With more than 80% of the ocean still unexplored, it remains a familiar feature of Earth with an interior that is largely unknown. What has been found so far includes the following discoveries, each reshaping what researchers thought they knew about the deep ocean.

Underwater Hot Springs Full of Creatures That Look Like They’re from Another Planet

An oceanography discovery that changed everything in 1977 saw scientists use the submersible Alvin to explore the Galapagos Rift off the coast of Ecuador. What they found were hot springs 2.5 kilometers deep along mid-ocean ridges. These ridges allow seawater to percolate into Earth’s crust, where it is heated by magma and expelled as mineral-rich superheated water. These hot springs, or hydrothermal vents, emit fluids that can reach temperatures as high as 400°C. The fluids also carry dissolved minerals, creating an environment disconnected from sunlight and unlike anything scientists expected.

Around the vents, an otherworldly ecosystem thrives. Giant tube worms, mussels, and clams exist in dense clusters. They do not need photosynthesis but rely on chemosynthesis, where bacteria convert sulfur compounds into energy. It is a startling sight, with chimneys rising from the seabed and plumes of scalding fluid. Ghostly invertebrates drift in complete darkness under immense pressure. This ecosystem challenged the long-standing assumption that all life needs sunlight and forced scientists to reconsider what is possible in such extreme environments.

Plastic Bags and Tiny Trash Pieces Lurking Even in the Deepest, Darkest Trenches

In a sad, but not surprising, turn of events, scientists documented plastic waste and fibers, including an entire plastic bag, from the deepest part of the ocean, the Mariana Trench. Researchers conducting deep-sea dives and analyzing the Deep-Sea Debris Database in 2018 confirmed that plastic pollution had reached the Challenger Deep in the trench. The plastic bag was at a depth of more than 10,000 meters below sea level, which is a startling indication of how far human waste can travel. It has long been known that human-generated waste sinks through the water column and is carried by currents until it ends up on the seafloor. The fact that plastic can settle at the deepest point of the ocean, where humans have barely explored, shows just how far-reaching this problem is.

Inevitably, this find raised concerns about the long-term ecological impact of continued plastic pollution. Deep-sea organisms already ingest microplastics, and these particles accumulate over time as decomposition slows in cold, high-pressure environments. Until a real solution to plastic pollution is found, marine life will remain at risk of entanglement and toxicity, making the presence of plastic at such depth even more unsettling.

A Golden Orb at the Bottom of the Ocean

Sometimes, scientists find something in the ocean that they cannot explain. In 2023, a NOAA Ocean Exploration team found what looked like a golden orb on the seafloor around 3,300 meters deep off the coast of Alaska. Upon closer inspection, the team found that the sphere was soft and leathery on the outside and only about 10 centimeters in diameter. They saw a hole on one side of the orb, which led them to believe it might be the remains of an egg case. The orb was attached to a rock, and white sponges surrounded it, but there were no nearby species, or any known species, that matched its appearance, making the find mysterious.

Using a remotely operated vehicle, the team removed the orb and had it analyzed; early results show it is organic. However, researchers still do not know exactly where the orb came from. Theories range from a deep-sea creature’s egg case to an unknown sponge or coral species, a level of uncertainty that surprised even experienced researchers.

The Remains of What Is Referred to as Japan’s Pearl Harbor

Some discoveries are not exactly a surprise, but they still come as a shock. In 1944, during WWII, the U.S. launched Operation Hailstone against Japan’s main naval base in what was then called Truk Lagoon or Truk Atoll. It took only two days for American forces to sink more than 40 ships and destroy hundreds of aircraft. Truk Lagoon was renamed Chuuk Lagoon in 1990 to honor the local culture, and the wrecks of the ships still lie here in an underwater graveyard. The locals in the area were aware of the site, but global attention grew after Jacques Cousteau and his team made a documentary in 1969, revealing the scale of the destruction to a wider audience.

Today, the sight of the wrecks remains eerie above all else. Divers can see the Fumizuki destroyer, the I-169 Kaidai-type submarine, and a 500-foot sunken tanker. The Shinkoku Maru’s sick bay and operating room floor are still covered in glass medicine bottles, and toilets and bathtubs sit visible beneath the silt. One of the most striking wrecks is that of the Heian Maru, a 510-foot passenger liner converted into a submarine tender. It landed on its port side after sinking and is only 115 feet deep. There are environmental concerns about the wreck site because some ships contain fuel and chemicals that could leak into the lagoon over time, a reminder of how unexpectedly persistent wartime damage can be.

A Fish That Makes the Most of Mirror Vision

The spook fish was first seen more than 120 years ago, when specimens were caught in the northern Pacific, but scientists only discovered that these fish use mirrors rather than lenses to focus light in their eyes. This is because the first live spook fish was only caught in 2008. The spook fish seems to have four eyes, but in reality, it has two eyes, each split into two linked parts. One half of each eye points upward, allowing the fish to see the ocean and food above it. The other half looks down into the abyss, an unusual adaptation for a vertebrate.

Microscopic analysis revealed that the eyes functioned as mirrors composed of stacks of crystalline plates. This discovery surprised researchers because almost all vertebrates rely on lenses, not mirrors, to focus light. Further analysis showed that the spook fish’s mirrors allowed it to direct and sharpen faint bioluminescence from the deep ocean, making its vision far stranger than expected.

Metallic Balls Covering the Seafloor Across the Abyssal Plains

Strange deep-sea creatures are not the only discoveries made in the depths of the world’s oceans. In 1873, the HMS Challenger, in the Kara Sea, came upon metallic-looking nodules on the ocean floor. At the time, the discovery was overlooked and considered unimportant. But when the HMS Challenger expedition also discovered and recovered nodules from abyssal plains in several oceans, attention suddenly shifted, and the expedition was given credit for the discovery, a change that highlighted how unexpected the find truly was.

The nodules, made of manganese, iron, nickel, and copper, still cover vast areas of the ocean floor. They grow imperceptibly slowly, with experts estimating only a few millimeters of growth over a million years. Mining companies expressed interest in the nodules because of the valuable metals they contain. However, researchers found that the spheres provided a habitat for deep-sea organisms. Sponges, corals, and microbes attach to them and use them as a stable base in a muddy environment, making their presence more significant than early explorers realized.

Upside-Down Waterfalls That are Taller Than Any Waterfall on Land

There have also been discoveries of phenomena typically associated with land and not the ocean. Oceanographers discovered the Denmark Strait underwater waterfall during mid-20th-century studies of water masses flowing between the Arctic and Atlantic oceans. While using instruments to measure temperature, salinity, and water density, they detected cold, dense Arctic water moving beneath the warmer Atlantic surface water. Mapping revealed that the dense water sank over an underwater ridge between Greenland and Iceland, forming a massive downward cascade, an effect far larger than researchers expected to find underwater.

The Denmark Strait cataract is not visible at the surface because it moves enormous volumes of water far below. The plunging cold water forms part of the deeper section of the world’s conveyor-belt current, where it helps regulate global ocean circulation, redistributes heat, and influences the climate. The flow depends on subtle density and temperature gradients, so scientists continue to monitor it as climate change alters seawater temperature and salinity, adding uncertainty to an already surprising phenomenon.

The Black Seadevil with a Living Flashlight on Its Head

On November 17, 2014, researchers recorded a live Black Seadevil in its natural deep-sea habitat while exploring the Monterey Canyon at a depth of around 1,900 feet. The cameras showed the fish’s glowing lure bobbing ahead of its wide, open mouth, swinging gently as the fish drifted in the dark water. The team also saw that the light lured prey within reach of the fish’s snapping jaws, a behavior rarely captured so clearly. It was an exciting observation, since it was a tactic long thought to exist in theory, but rarely seen in action.

The black seadevil uses its lure to hunt in almost total darkness. The light is created by symbiotic bacteria living in the lure’s tip. The fish angles or waves it to mimic prey or attract curious sea animals. The female grows large, with a wide mouth and inward-curving teeth ideal for capturing prey nearly her own size. The male stays tiny and lacks a lure. Males attach themselves to females, fusing with them and sharing their bloodstream. This is a strategy known as sexual parasitism, a striking example of how unusual deep-sea survival can be.

Hadal Trenches Known as Earth’s Last Frontier

Once hadal trench exploration expanded with deep-diving submersibles and unmanned landers, explorers kept pushing boundaries. Over several decades, missions to trenches deeper than 6,000 meters, including the Mariana, Tonga, and Japan trenches, helped retrieve sediment samples, capture video footage, and collect small creatures. Scientists found abundant life in the deep, including sea cucumbers, microbes, snailfish, and amphipods, at a surprising level of diversity. These creatures adapted to crushing pressure and cold temperatures and thrived in the darkness.

They rely on slow metabolism and flexible bodies to survive. Their specialized biochemistry allows them to endure conditions once thought uninhabitable, with some species appearing only in the trenches and nowhere else, underscoring how unique life can be at extreme depth. The hadal zone remains one of the least explored habitats on Earth, with many trenches sampled only once or twice. As technology advances, scientists expect to learn more about how life persists in these remote environments.

The Giant Phantom Jellyfish Drifting Silently in the Deep

Researchers at Monterey Bay Aquarium Research Institute captured the first clear video of a giant phantom jellyfish at around 990 meters in Monterey Bay in 2021 during a deep-sea survey. The footage shows a dark-bodied, gigantic jelly drifting in midwater zones between 990 and 6,700 meters deep. Researchers noted that the jelly has a broad bell over a meter wide and long, ribbon-like arms trailing behind it, undulating slowly as it moved, a rare view of a species seldom seen alive. Phantom jellyfish inhabit remote depths and lack hard parts, making net sampling very difficult. Video footage is the best researchers could hope for.

They also found that the phantom jellyfish survives by drifting and capturing slow-moving prey. Its arms act almost like sticky nets that trap plankton and tiny fish. These jellyfish conserve energy by not swimming quickly, an advantage in a food-scarce environment. Scientists still do not know much about the phantom jelly’s life cycle, diet, or reproduction, leaving the species mysterious despite the recent footage.

The Deep Ocean Is a World Still Waiting to be Fully Understood

The deep ocean is still one of the most puzzling places on Earth, a place where imaginations run wild and discoveries exceed expectations. Every strange or surprising find reaffirms how much we still do not know about the vast world beneath the waves. While most of the ocean remains unexplored, its depth continues to reveal unexpected details, and there is no doubt that even stranger discoveries await. As scientists push for refined and updated technology, each new descent adds to a picture that is far from complete, changing how we understand and view our planet and its oceans.