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Go to Brilliant.org/SciShow to check out their Knowledge and Uncertainty course. {♫Intro♫}. Underneath kilometers of seawater, the ocean floor is full of riches.

There's gold, iron, and lots of other rare, precious metals. And no, it doesn't have anything to do with pirates. All this treasure is the result of millions of years of geochemical processes that have been steadily filling the seafloor with loot.

Some of the most impressive deposits grow at hydrothermal vents, which are hot springs at the bottom of the ocean. These vents form along mid-ocean ridges, where Earth's tectonic plates are splitting apart, and lava rises up through the gap, then hardens into long underwater mountain chains. At these spots, seawater seeps into cracks in the ocean crust and collects underground, where it gets heated by magma and bursts out through the vents.

But it's not pure seawater that comes back out. The whole time it's underground, the hot seawater is leaching metals from the surrounding crust, which is full of huge amounts of sulfur, along with metals including copper, gold, zinc, lead, and silver. So when that hot water gushes back out from the vent, it interacts with the cold water around it, and all those dissolved metals precipitate back out of the liquid.

Over time, they form tall, chimney-like structures that rise up around the vent. Sometimes the chimneys are called white or black smokers because the erupting fluids full of minerals look like smoke rising out of the towers, but the whole accumulations are known more formally as massive sulfide deposits. These can only form under the ocean, but these days, when we mine for zinc, copper, or silver on land, we're often mining massive sulfide deposits that originally formed on the ocean floor millions of years ago.

It's just that, since then, they've ended up above sea level thanks to the shifting of tectonic plates. For example, the island of Cyprus has 30 of these massive sulfide deposits that were once under the sea, and that's where the Ancient Romans got their copper. Unlike hydrothermal vents, though, some underwater deposits accumulate much more subtly, such as the ones that grow on exposed ridges, plateaus, and underwater mountains left behind by extinct volcanoes.

As seawater comes in contact with these surfaces, metals dissolved within it precipitate out to form a crust over the rocks. It all starts when iron and manganese oxides latch on to the rock. They're prone to chemical reactions and have a lot of surface area, which makes them especially sticky as far as chemicals go.

So they'll glom on to any exposed rock surface. Then, once these compounds build up enough, they form a sponge-like network that traps other metals from the seawater in the pores. Seawater itself has pretty low concentrations of metals, but as those metals collect on exposed surfaces, they can reach concentrations 1 billion times higher than in the ocean.

These so-called cobalt-rich crusts contain metals like cobalt, iron, and manganese, as well as rare earth metals like tellurium and platinum. And depending on the strength and direction of the ocean currents, the crust can form just a glaze, or it can grow to about 25 centimeters thick. But these crusts grow incredibly slowly.

Every million years, they accumulate just one to 10 millimeters. It's one of the slowest processes on Earth. Finally, the last process we're going to look at in our tour of the seafloor produces what might be the oddest of our ocean treasures: a potato-shaped clump made of minerals containing manganese, iron, and smaller amounts of other metals.

These lumpy rocks are known as manganese nodules, and they grow in underwater fields more than 3500 meters below the surface. Each nodule gets its start as a piece of debris, like a shark tooth or a pebble, that serves as a nucleus. Then, little by little, manganese and iron oxides attach to this debris just like they attach to exposed rock.

And over millions of years, the metals grow in concentric rings, kind of like a jawbreaker candy. All this iron and manganese comes from seawater and also from liquids that squeeze out from pores in the sediments beneath the rocks. Scientists think that the nodules also get a little help from special bacteria that live on them.

They believe these microbes speed up the process by taking manganese from the seawater, converting it to manganese oxide, and depositing it on the clump. These odd treasures were discovered in 1873, during the first-ever worldwide oceanographic expedition. The crew of that expedition dragged up a few lumpy rocks from the seafloor and were surprised to find that they were big hunks of manganese oxide.

Today we know that there are thousands of square kilometers of these nodules growing on the seafloor, and they contain more manganese than there is anywhere on land. Since their discovery, these seafloor treasures have generated a lot of interest, both from scientists and mining companies. Many of these metals are in short supply on land, but are in demand for manufacturing high-tech electronics, like cell phones and rechargeable batteries.

But since the ore lies beneath hundreds of meters of water, no one is mining these deep sea treasures—yet. Which is a good thing, because we don't fully understand how that would alter seafloor ecosystems, or the larger ocean environment. After all, as common as they are, the ocean floor has been growing these treasures for millions of years, so we know they don't come easy, which gives us all the more reason to protect them.

There's still so much to learn about what goes on under the ocean, and sometimes it can feel overwhelming when you realize just how much we don't understand. But if you've ever felt that way, you might enjoy Brilliant's course on Knowledge and. Uncertainty.

You can learn how to put a number on uncertainty—and how to minimize it. Brilliant also has a wide selection of other courses in science, engineering, math, and computer science, and if you've been looking for a new, productive way to spend your time at home, you can see what Brilliant has to offer. And if you're one of the first 200 people to sign up at Brilliant.org/SciShow, you'll get 20% off an annual Premium subscription.

So check it out if you're interested, and as always, thanks for watching SciShow. {♫Outro♫}.