What’s the 2nd Deepest Trench (After the Mariana Trench)? And How Much Deeper is the Mariana Trench? Also, Just What are Ocean Trenches Anyway?

Perhaps you’ve heard of the Pacific Ocean’s deepest trench—the Mariana Trench—but what’s the next most deep ocean trench? How deep is that one compared to the Mariana Trench?

Before that, though, I’m going to address the question of what ocean trenches even are. If you know geology, you know one of the things that can happen where two tectonic plates come together is for them to push against each other. And when this happens, the heavier one will get pushed underneath the lighter one. At the point of this descent, a trough forms—and these troughs are called trenches, many of which occur in the earth’s oceans.

https://commons.wikimedia.org/wiki/File:Cross_section_of_mariana_trench.svg Illustration of the Mariana trench by Fryer Hussong & redrawn by Vanessa Ezekowitz, CC BY-SA 3.0

The deepest, lowest locations on our planet lie in ocean trenches. As mentioned, the Mariana Trench is the deepest ocean trench on Earth. And Challenger Deep (located in the Mariana Trench) is specifically the deepest point we know about. In case you’re wondering where on Earth it is, the Mariana Trench lies east of the Philippines, south of Japan, but north of Papua New Guinea (and it’s over 1,500 miles long) with the Challenger Deep point reaching a depth of approximately 36,200 feet (or about 11,034 meters) below sea level and positioned about 200 miles southwest of Guam.

The next deepest place—the Tonga Trench—is in the Southern Hemisphere (…and—more trivia—it’s the Southern Hemisphere’s deepest location) lying in the southwest Pacific Ocean between New Zealand and Tonga. This ocean trench also has the most rapid tectonic movement of any of the ocean trenches. The deepest point within the Tonga Trench—Horizon Deep—reaches about 35,509 feet (or around 10,823 meters) below sea level.

Deep Ocean photograph of fishes and sun looking up through the water; image by PublicDomainPictures from Pixabay https://pixabay.com/photos/animal-asia-blue-dark-deep-depth-18719/

So, to address that question of how much deeper the Mariana Trench is than the Tonga Trench, I’ll calculate a bit of math. Specifically, 36,200 – 35,509 = 691 (or 11,034 – 10,823 = 211 if you prefer meters to feet). However, note that I’m using measurements from Challenger Deep and Horizon Deep, which are within the two trenches (because each are the maximum depth points for these two trenches). Which means that technically, Challenger Deep is 691 feet (or 211 meters) deeper down than Horizon Deep. But I suppose you could still generally say the Mariana Trench is 691 feet (or 211 meters) deeper than the Tonga Trench.

Now to throw a wrench in all that, though. Did you notice how I mentioned that the fastest tectonic movement is currently occurring along the Tonga Trench? This means that although right now Challenger Deep is deeper than Horizon Deep, that could change over time… 😉

Underwater fish and seafloor; image by Rudy and Peter Skitterians from Pixabay https://pixabay.com/photos/fish-underwater-diving-scuba-diving-384629/

To round out the top five, Emden Deep (AKA Galathea Depth) in the Philippine Trench, east of Micronesia and north of the Solomon Islands, measures 34,580 feet (or 10,540 meters) below sea level, the Kuril-Kamchatka Trench, located off the southeast coast of Kamchatka, reaches a depth of 34,449 feet (or 10,500 meters), and the Kermadec Trench (northeast of New Zealand’s North Island) comes in at 32,963 feet (or 10,047 meters) deep. Which means every ocean trench in the top five is located in the Pacific Ocean.

Finally, for fun, let’s compare the world’s highest mountain—Mount Everest—to the world’s lowest valley—Challenger Deep. Mount Everest comes in at 29,032 feet (8,849 meters) tall while Challenger Deep reaches about 36,200 feet (11,034 meters) down. If each were inverted, how much higher/deeper would Challenger Deep be? Well, 36,200 – 29,032 = 7,168 feet (or 2,185 meters). So not only would Mount Everest be a whole lot lower/shallower than Challenger Deep, but it would also be shorter than every one of the top five deepest points.

Palm trees on a sandy ocean beach; image by Sasin Tipchai from Pixabay https://pixabay.com/photos/beach-palm-trees-sea-seascape-1822544/

As for me, I think I’ll stick to shallow beaches for my summer water fun! 🙂