The oceans cover over 70 percent of our planet, so it's little wonder that their seemingly impenetrable depths have provided a series of fascinating mysteries, from missing ships to eerie monsters. Below are six mysteries of the deep—some of which scientists think they've at least partly explained, while others remain truly puzzling.
The Mary Celeste
On December 5, 1872, the crew of the British ship the Dei Gratia spotted a vessel bobbing about 400 miles off the coast of the Azores. They approached the Mary Celeste to offer help, but after boarding the ship were shocked to find it completely unmanned. The crew had disappeared without a trace, their belongings still stowed in their quarters, six months' worth of food and drink untouched, and the valuable cargo of industrial alcohol still mostly in place. The only clues were three and a half feet of water in the hold, a missing lifeboat, and a dismantled pump. It was the beginning of an enduring mystery concerning what happened to the crew, and why they abandoned a seemingly sea-worthy vessel.
Numerous theories have been suggested, including by crime writer Arthur Conan Doyle, who penned a short story in 1884 suggesting the crew had fallen victim to an ex-slave intent on revenge. A more recent theory has pointed the finger at rough seas and the broken pump, arguing they forced the captain to issue an order to abandon ship. Since the missing crew have never been traced, it seems unlikely that there will ever be a satisfying answer to the enigma.
The Yonaguni Monument
In 1986, a diver looking for a good spot to watch hammerhead sharks off the coast of the Ryukyu Islands in Japan came across an extraordinary underwater landscape. The area reportedly looked like an ancient submerged village, with steps, holes, and triangles seemingly carved into the rocks. Ever since it was first discovered, controversy has surrounded the site that's become known as the Yonaguni Monument, with some researchers—such as marine geologist Masaaki Kimura—arguing it is a clearly manmade environment, perhaps a city thousands of years old and sunk in one of the earthquakes that plagues the region. Others believe it's a natural geological phenomenon reflecting the stratigraphy (layers) of sandstone in an area with tectonic activity. The area is open to scuba divers, so the really curious can strap on air tanks and decide for themselves.
The Bermuda Triangle
The Bermuda Triangle has probably spawned more wild theories, column inches, and online discussion than any other ocean mystery—more than 50 ships and 20 aircraft are said to have vanished there. Although the triangle has never officially been defined, by some accounts it covers at least 500,000 square miles and lies between Bermuda, Florida, and Puerto Rico.
The mystery first caught the public imagination in December 1945 when Flight 19, consisting of five U.S. Navy TBM Avenger torpedo bombers and their 14 crewmembers, were lost without a trace during a routine training operation in the area. Interest was further piqued when it was later reported that one of the search-and-rescue planes dispatched to find the missing team had also disappeared. Articles and books such as Charles Berlitz’s The Bermuda Triangle, first published in 1974 and having since sold over 20 million copies in 30 languages, have served to keep the mystery alive, providing potential theories both natural and supernatural. Scientists—and world-renowned insurers Lloyd’s of London—have attempted to debunk the myth of the Bermuda Triangle, offering evidence that the rate of disappearance in the vast and busy triangle is no higher than other comparable shipping lanes, but such is the power of a good story that this is one story that seems likely to continue to fascinate.
For hundreds of years, sailors told tales of an enormous sea creature with huge tentacles known as the Kraken. Stories around the mythical kraken first started appearing in Scandinavia in the 12th century, and in 1555 Swedish cartographer Olaus Magnus provided an account of a sea creature with “sharp and long Horns round about, like a Tree root up by the Roots: They are ten or twelve cubits long, very black, and with huge eyes.” The stories persisted, often mentioning a creature so large it resembled an island. In his 1755 book The Natural History of Norway, Danish historian Erik Ludvigsen Pontoppidan described the kraken as “incontestably the largest Sea monster in the world."
Scientists have proposed that these stories might derive from sightings of giant squid (Architeuthis dux), although evidence for an even larger, yet extremely elusive, colossal squid (Mesonychoteuthis hamiltoni) has also come to light. The colossal squid is found in the deepest part of the Southern Ocean surrounding Antarctica, and is thought to be up to 46 feet long and 1100 pounds. The problem is that the animal is so rare very few specimens have been found intact, and no live specimen has ever been observed, which means that estimating its exact size is difficult. Researchers have also noticed that sperm whales have been observed with large scars, and have suggested that these could be the result of violent encounters with the colossal squid, which is known to have sharp rotating hooks on the ends of their tentacles.
The Treasure of the Merchant Royal
The remains of the Merchant Royal are known as one of the richest shipwrecks ever. The ship set sail from the New World in 1641 laden with 100,000 pounds of gold, 400 Mexican silver bars, and thousands of precious gems—in total, a haul thought to be worth $1.3 billion today. The ship got caught in a storm and was thought to have gone down somewhere off the coast of Cornwall, England. The lost wreck became known as the “el Dorado of the seas” due to the enormous value of its cargo, and over the years numerous treasure hunters have searched fruitlessly for its final resting place, which remains undiscovered. In 2019 fishermen snagged what is thought to be the anchor from the Merchant Royal, but to date the dangerous conditions and extreme depths at which the wreck is thought to lie have meant it has remained unclaimed.
Attack of the Sea Foam
In December 2011, residents of Cleveleys, England, awoke to what appeared to be a soft blanket of snow. But as locals ventured out into the streets it soon became clear that this was no snowstorm, but instead something far more puzzling. Trees, cars, roads, and houses were all wrapped in a thick, white layer of foam. The Environment Agency were quickly deployed to take samples of the sea foam, since residents were understandably concerned as to the origin of the strange, gloopy substance, fearing it might be caused by pollutants.
The dramatic images of the foam-soaked town soon had journalists flocking to the region to investigate the phenomena, but as quickly as it appeared the foam disappeared, leaving behind only a salty residue. Scientists analyzing the foam confirmed it was not caused by detergents, and instead suspected that it was caused by a rare combination of decomposing algae out at sea and strong winds, which whipped up the viscous foam and blew it into land. The phenomena has apparently occurred at other times before and since, and researchers are now working to try and understand the exceptional conditions that cause it to form so that residents can be warned when another thick blanket is set to descend.
Bonus: The Bloop—Mystery Solved
Over the years, the oceans have produced a number of eerie and often unexplained sounds. In 1997, researchers from NOAA listening for underwater volcanic activity using hydrophones (underwater microphones) noticed an extremely loud, powerful series of noises in the Pacific Ocean. The unusual din excited researchers, who soon named it “The Bloop” in reference to its unique sound.
Theories abounded as to the origin of the bloop—secret military facility, reverberations from a ship’s engine, or an enormous sea creature. The most fanciful suggestion stem from H. P. Lovecraft fans who noticed that the noise came from an area off South America where the sci-fi writer’s fictional sunken city of R’lyeh was supposed to be. They proposed that the bloop might have originated from Lovecraft’s “dead but dreaming” sea creature, Cthulhu. In 2005, however, scientists found that the mysterious sound was in fact the noise made by an icequake—or an iceberg shearing off from a glacier.