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Place 04 · The Ridge and the Trench · Wrap-up
The Floor of the World
The bottom of the ocean turns out to be young, restless, crushingly heavy and alive, and right now people are arguing over who gets to dig it up.
About 20 minutes · 1 video · a full practice review of the whole seafloor
Look back
Seven chapters ago you did not know this
You can now
Map a place nobody can see
Time a sound’s echo, multiply by 1,500 m/s and halve it. Enough pings, and Marie Tharp’s maps revealed a 65,000 km ridge with a rift along its crest.
You can now
Read the seafloor’s barcode
Lava locks in Earth’s magnetic field, the field flips, and mirror-image stripes on both sides of the ridge prove the seafloor spreads. It is young: no more than about 180 million years in the open ocean.
You can now
Find where the seafloor goes
Plates pull apart at ridges, grind past at transform faults and dive back into the mantle at trenches like the Mariana and the Peru–Chile.
You can now
Explain a tsunami
A megathrust earthquake lifts the seafloor, and a wave hundreds of kilometers long races across the ocean at jet speed, then slows and rises at the coast.
You can now
Feel the weight of the water
One more atmosphere every 10 m, more than 1,000 at the Challenger Deep. Air spaces get crushed; watery animals and TMAO-packed fish cope.
You can now
Explain life without the Sun
At hydrothermal vents, microbes make food from hydrogen sulfide, and tubeworms with no mouth or gut live off the bacteria inside them.
You can now
Read the mud
Sediment comes from land, life, seawater and space, piles up in layers that record past climates, and grows slow, metal-rich nodules.
Photos: gang chen, Errol Briant, Jess Loiterton, Max J, Francesco Ungaro, Diego Sandoval, Valentin Onu / Pexels
One more thing
Who owns the bottom of the sea?
NOAAPMEL — Our Deepest Waters: Marianas Trench, Marine National Monument. Five and a half minutes from NOAA scientists on what is being protected around the deepest place on Earth, and why.
The seafloor by the numbers
- 54% — of the ocean floor lies in the Area, beyond any country’s jurisdiction
- 172 — members of the International Seabed Authority (171 countries plus the European Union), as of February 2026
- 2009 — Marianas Trench Marine National Monument created, about 246,600 km² of ocean and seafloor
- 28.7% — of the seafloor mapped in detail as of 2026: most of what might be mined is barely charted
More than half of the ocean floor, about 54 percent, lies beyond any country’s jurisdiction. Under the UN Law of the Sea that seafloor is called the Area, its minerals are the “common heritage of humankind,” and the International Seabed Authority decides who may explore it and, one day, mine it. The nodules from Chapter 7 hold nickel, cobalt and copper for batteries, so companies want to start; but tracks from a 1989 plowing experiment in the Peru Basin were still visible 26 years later, and more than 40 countries have called for a pause. As of September 2026 the ISA’s mining rulebook is still unfinished and nobody has been allowed to mine the Area commercially, while the United States reviews a mining application under its own law; yet over its own deepest waters, back in 2009, the US chose protection: the Marianas Trench Marine National Monument.
Figures: International Seabed Authority, NOAA Fisheries and Seabed 2030.
Two pictures to leave with
What the deep floor is made of, and where the deep water comes from
Put it all together
Seafloor Review
Seven chapters, one seafloor. This review walks the whole place, from the first sonar ping to the mud and the metal, with a few questions that tie chapters together. Retry as often as you like. If one trips you up, the feedback names the chapter to revisit, and every chapter has its own eight-question practice set at the bottom of the page.
Practice
Seafloor Review: the whole place
Twenty questions covering all seven chapters and this page. Aim for a clean run, then you are ready to come back up.
Where we go next
Next stop: The Southern Ocean
All that cold, dark water you just dived through has to come from somewhere, and a lot of it starts around Antarctica. When seawater freezes into sea ice, most of the salt is left behind in the water below, which becomes so cold and salty that it sinks all the way to the bottom and creeps out across the deep basins you just explored. Place 05 goes to the ocean around Antarctica to watch it happen.
Photo credits (Pexels): Kris Møklebust, gang chen, Errol Briant, Jess Loiterton, Max J, Francesco Ungaro, Diego Sandoval, Valentin Onu, Arina Krasnikova.