1. Gulf of Mexico Shipwreck
The Gulf of Mexico holds thousands of wrecks, but one early-19th-century vessel found more than 2,600 feet down has been unusually stubborn. ROV dives have been dogged by technical glitches, corrupted video and equipment failures, feeding a superstition that the site wants to stay secret. Archaeologists are still piecing together the ship’s identity, cargo and final voyage.
2. Colossal Squid
The colossal squid (Mesonychoteuthis hamiltoni) is even bulkier than the giant squid. It can exceed 10–14 metres in total length and sports rotating hooks on its tentacles. Very few mature specimens have been studied, so questions remain about its behaviour, lifespan and true maximum size. For background on deep-sea cephalopods and other ocean science, see Smithsonian Ocean.
3. Leptocephalus giganteus
First described in 1959 by ichthyologist Peter Henry John Castle, this enormous “leptocephalus” was likely the larval stage of an eel. If the larva was genuinely from a typical growth cycle, then the adult could have been vast, which sparked debate. Most specialists now suspect the measurements were unusual or misinterpreted, but the find still fuels legends of giant eels in the abyss.
4. Aegean Sea Ruins
Off Delos, Greece, divers uncovered an extensive seabed complex in 2014. What looked like a port turned out to include a kiln, paved features, retaining walls and building outlines across roughly 12 acres. Was it a coastal workshop sunk by seismic activity or gradual subsidence? The precise date and cause are still under study, adding to the Aegean’s long maritime mystery list.
5. Cuban Underwater “City”
In 2001, sonar imaging near Cuba revealed geometric forms at depths of 600–750 metres. Some observers saw pyramids and avenues, while others argued for natural rock patterns and artefacts of sonar processing. The area’s depth and currents make verification difficult, so the true story of these shapes remains unresolved.
6. Baltic Sea Anomaly
Discovered by Swedish treasure hunters in 2011, this roughly 60-metre object on the Baltic floor has been called everything from a crashed craft to a glacial formation. Most geologists lean toward a natural explanation, but the oddly regular contours and “track” signatures in some scans keep the conversation alive.
7. The Dragon’s Triangle
Also known as the Devil’s Sea, this area south of Japan has a reputation for disappearances, much like the Bermuda Triangle. Explanations range from undersea volcanism and methane hydrate releases to strong currents and severe weather. For a sober overview, see Britannica’s entry on the Devil’s Sea.
8. Atlantis
Plato’s Timaeus and Critias describe a sophisticated island power that vanished in a day and a night. Was it a moral allegory, a memory of real Bronze Age catastrophes, or pure invention? Claims linking Atlantis to locations from the Mediterranean to the Atlantic crop up regularly, but conclusive archaeological evidence has never been found.
9. Megalodon
The Megalodon (Otodus megalodon) was a gigantic prehistoric shark, likely topping 15–18 metres. Why it vanished about 3.6–2.6 million years ago is still debated. Cooling oceans, shifting prey ranges and competition from ancestors of great white sharks all feature in the leading theories. Bite-marked fossils show it was a powerful predator, yet its exact hunting strategy remains a puzzle. Explore expert summaries at the Natural History Museum.
10. Bimini Road
Near the Bahamas, a neat line of limestone blocks lies in shallow water. Some see a paved causeway and link it to Atlantis. Most geologists explain the pattern as beachrock that fractured and eroded in straight lines. Until an ancient quarry, tools or associated settlement is found, Bimini Road will sit on the boundary between natural oddity and human handiwork.
The deep sea is a frontier where nature often outstrips our imagination. Whether these mysteries resolve into geology, lost history or new biology, each discovery helps us understand our blue planet a little better.
Join the Conversation
Which of these mysteries captured your imagination the most? Do you know another deep-sea enigma we should add to a future list? Tell us in the comments below, then keep exploring our latest lists from the The Very Best Top 10 homepage and our sea-search results. If you enjoyed this, share it with a friend and bookmark us for your next curiosity fix.