The Mariana Trench is universally recognized; it’s the subject of documentaries and mentioned in school textbooks. However, our planet is far more intricate than a solitary record point in the Pacific Ocean. Surrounding the Mariana Trench lies an entire array of abysses—each one deeper than Mount Everest, every one harboring its own unique history, anomalies, and chilling secrets. We will explore five trenches that receive little notice but truly deserved to be oceanographic headliners if they weren’t overshadowed by their more famous neighbor.
Beyond the Mariana: 5 of the Planet’s Most Terrifying Undiscovered Abysses
The Tonga Trench: Plutonium on the Seabed
In the southwestern Pacific, east of the eponymous archipelago, the Tonga Trench stretches for roughly 860 kilometers. It ranks as the second deepest trench globally, second only to the Mariana, and simultaneously holds the deepest point in the entire Southern Hemisphere. This area is called the Horizon Deep, plunging to 10,800 meters—over ten kilometers of water pressing down from above.
Yet, it’s not this depth that truly makes the trench unnerving. In 1970, following the failed launch of the Apollo 13 mission, a Radioisotope Thermoelectric Generator (RTG)—a power source running on Plutonium-238—fell into the Pacific. The isotope’s activity totaled approximately 44,500 curies. The generator sank near the Tonga Trench and remains there today. NASA monitored the regional radiation levels, reporting no release; the casing apparently withstood the pressure and did not breach. Officially, everything is contained.
Unofficially, however, nobody has retrieved the object, nor has anyone inspected its condition up close. What exactly has happened to the metallic housing under hundreds of atmospheres of pressure over half a century remains unknown. The Tonga Trench stays one of the planet’s least-explored deep-sea trenches, having seen far fewer expeditions than the Mariana. Immense depth, seismic activity, and potentially a radioactive relic from the space race—this is what lies at the bottom of this abyss.
The Philippine Trench: Strangely Asymmetrical
East of the Philippine Islands, stretching from Luzon in the north down to the Maluku Islands in the south, lies the Philippine Trench, spanning 1,320 kilometers and reaching depths of up to 10,540 meters. For many years, it was known by another name: the Mindanao Trough, named for the large island near which it is situated.
This trench provides a textbook example of subduction in action. The Philippine Sea Plate is physically slipping beneath the Eurasian Plate at a rate of about 16 centimeters annually. While this seems minor, in geological terms, it means oceanic crust is continually swallowed by the mantle to depths of 50–100 kilometers, where it melts under incredible pressure and heat. This very mechanism creates the trench—like a fold appearing on a tablecloth being pulled from one end.
One defining feature of the Philippine Trench is the asymmetry of its slopes: the western and eastern walls exhibit fundamentally different angles of inclination, which itself sparks scientific debate. The German research vessel ‘Planet’ was the first to conduct detailed surveys across the entire length of the trench in 1912—a surprisingly early date for such a complex feature. Since then, the trench remains a zone of heightened seismic risk and a key source of data concerning the mechanics of subduction zones in the western Pacific.
The Kermadec Trench: Giant Amphipods and Fish Out of Place
Located in the southwestern Pacific, northeast of New Zealand, the Kermadec Trench runs for about 1,200 kilometers, smoothly merging with the Tonga Trench to the north, forming one of Earth’s longest deep-sea systems. Its maximum depth registers at 10,047 meters.
The trench was discovered in 1889 by the British ship ‘Penguin,’ but precise depth soundings were taken by the Soviet research vessel ‘Vityaz’ in 1958—that same ‘Vityaz’ which explored other deep trenches in the Pacific and became a true legend of Soviet oceanography.
However, the Kermadec’s main mystery is biological. In 2012, giant amphipods of the species Alicella gigantea were found on the trench floor, reaching lengths of up to 34 centimeters. To put this into perspective, a typical member of this species grows to only about 2 centimeters. That’s a seventeenfold difference in size. This phenomenon has been termed “deep-sea gigantism,” and scientists still haven’t reached a consensus on its cause: is it due to the cold temperatures, the immense pressure, or specific feeding adaptations at great depths?
Even more inexplicable is the case of the pearlside fish Echiodon neotes, caught here at depths between 8,200 and 8,300 meters. All other known members of this genus inhabit depths of only 1,800–2,000 meters. How and why this fish ended up four times deeper than its usual range remains a question science has yet to answer.
The Kuril-Kamchatka Trench: Russia’s Abyss with a Literary Past
Along the eastern slopes of the Kuril Islands and Kamchatka Peninsula runs one of the longest trenches globally—the Kuril-Kamchatka Trench, extending approximately 2,170 kilometers. According to the Great Russian Encyclopedia, its maximum depth is 9,717 meters. This places it fifth deepest worldwide—yet it is one of the least familiar to the general public.
The trench once bore a different name: the Tuscarora Deep. This name arose in the 1870s when an American expedition laying a telegraph cable between San Francisco and Yokohama unexpectedly discovered significant depths beneath them. The cable was laid, the data recorded—and interest in the trench subsequently waned for a long time. For nearly 75 years, it remained obscure, mentioned only in specialized sailing guides.
It was during this period of neglect that the Tuscarora Deep unexpectedly entered literature. In 1944, the Soviet science fiction writer Ivan Yefremov published his short story “Encounter Over Tuscarora”—a tale about ships meeting above this very abyss. Yefremov was a paleontologist by training and knew what he was writing about.
Serious scientific investigation only commenced in the 1950s when the Soviet ‘Vityaz’ conducted comprehensive studies. It was revealed that this wasn’t just a deep scar in the ocean floor, but an epicenter for powerful earthquakes—this is where the Pacific Plate subducts beneath the Okhotsk Sea Plate, a process that frequently triggers major earthquakes and tsunamis in the Russian Far East. The trench lies right on Russia’s doorstep, yet it is incomparably less studied than its Pacific counterparts.
The Puerto Rico Trench: The Atlantic’s Deepest Abyss Close to New York
All four preceding trenches are situated in the Pacific Ocean—the deepest and most “trench-ridden” of all. But the Atlantic certainly has its own abyss. The Puerto Rico Trench runs along the boundary between the Atlantic Ocean and the Caribbean Sea and represents the deepest point in the entire Atlantic at 8,376 meters.
The deepest point of the trench bears a dual name: the Milwaukee Deep, also known as the Brownson Deep. It was charted in 1939 by an American naval vessel—a noteworthy fact, as military ships often made crucial oceanographic discoveries simply because they spent extended periods at sea equipped with echo sounders.
The trench is formed at the convergence of the Caribbean and North American tectonic plates. This is an active seismic zone: it was here that the catastrophic 2010 Haiti earthquake occurred, its focus located in the immediate vicinity of this tectonic boundary. The trench is perpetually “alive”: plates move, stress accumulates, and occasionally releases through seismic tremors.
The main contrast for this trench is geographical. All other abysses on this list are located in remote corners of the Pacific Ocean, far from major settlements. The Puerto Rico Trench is situated roughly 1,500 kilometers from New York City. It is the most “civilized” deep-sea location on the planet: the nearest metropolis is a few hours’ flight away, yet beneath the water’s surface lies over eight kilometers of darkness, pressure, and near totality of the unknown.
Conclusion
These trenches are more than just entries in a record book. At the bottom of each, immense pressure should preclude all life—yet something invariably thrives there: giant crustaceans, microorganisms, and even fish. The hadal zone—below 6,000 meters—remains one of the last truly unexplored frontiers on Earth.
While the Mariana Trench justly claims the title of deepest, its five less-publicized counterparts are far more mysterious and compelling—each in its own distinct way.
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