The Mariana Trench

Misha Glenny (0:00)

This is In Our Time from BBC Radio 4, and this is one of more than 1000 episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. Hello. In 1875 in the Western Pacific, the crew of HMS Challenger discovered the Mariana Trench, which turned out to be deeper than Everest is high by some two kilometres. Now trenches like Mariana form when one tectonic plate slips under another and heads downwards towards the Earth's mantle, and there are around 50 of them globally. Now some people used to think that it was too dark and deep for life to exist there. Others imagined monsters lurking at the bottom of the ocean. The truth has proved to be more intriguing than either of those. With me to discuss the Mariana Trench are three people who were all veterans of this kind of environment. Alan Jamieson, Director of the Deep Sea Research Centre at the University of Western Australia, John Copley, Professor of Ocean Exploration and Science Communication at the University of Southampton, and Heather Stewart, Director of Kelpie Geoscience and Associate Professor at the University of Western Australia. Heather, I'd like to come to you first. Can you just describe the Mariana Trench to us? How big is it? Where is it? And if we could see it, what would it look like?

Heather Stewart (1:41)

Yeah, fantastic. The Mariana Trench is what's called a subduction trench. And what it looks like is this long, curved deep within the Western Pacific. And that's formed as you introduced through a process of plate tectonics. So we have denser oceanic lithosphere, so the Pacific Plate, that plate encompasses the entire Pacific Ocean. And that is being thrust and pulled underneath the adjoining continental plates. So this process of plate tectonics by which these oceanic plates are getting taken down into the mantle and recycled, that downward flexure causes these ultra deep parts of our world, most famously the Mariana Trench and the other trenches that surround the Pacific, the so-called Pacific Ring of Fire. And that's how these ultra deep places are formed. And the maximum depth of the Mariana Trench is 10,925 metres.

And that's about 98 times the height of St. Paul's Cathedral, which is just, it's a really hard number to sort of visualise in your head when you start to think about these sort of deep water environments.

Misha Glenny (2:52)

And how long is it exactly?

Heather Stewart (2:54)

It's about 2,550 kilometres long, and it sort of arcs around the Mariana Isles in the Western Pacific there.

Misha Glenny (3:04)

Now, you're all experienced divers in these terrains. Heather, what's it like to go down a trench?

Heather Stewart (3:12)

It's absolutely incredible. I mean, all three of us around the table here have been in submersibles. But from my personal point of view, there's the moment when you're sitting on the sea surface and you get the clear to dive call. And that colour change as you start to fall through the water column and the change from the sort of clear waters on the sea surface through the brightest shades of blue down to absolute pitch blackness. But then, of course, all of that, you're sitting in silence. And that is so humbling as well as very, very exciting. Because of course, after a few hours, you start to come to the sea floor in these deep subduction trenches. And I've been lucky enough to dive to the bottom of the Tonga Trench. But that moment when you turn on the lights of the submersible, and you start to see the sea floor coming up underneath you is absolutely fantastic. And as a geologist, knowing that you're the first person to set eyes on this seascape, if you will, but also starting to look, and your brain is already starting to process what you're seeing out of the viewports and trying to put that into some sort of geological context. You know, are we landing on soft sediment sea floor or are we coming down on rocks? What type of rocks are there? Are there any structure in those rocks? Are we seeing faults? You know, what life is encrusting and are being associated with that habitat down there? So, you're constantly taking this information in and trying to form a hypothesis and that you're testing during the submersible dive itself. But I mean, it's absolutely, you know, the very first dive I did, the pilot sort of joked that, you know, you had to turn up the oxygen because I was getting very excited. So, I was using up more oxygen in the environment inside the sub. But it's truly, you know, that being there and sort of seeing it yourself is something that can't be replicated through other means.