“Why is the sky blue?” is, like, the classic curious-kid question.

People wonder about it all the time. But for all those people asking why the sky is blue, for some reason you don’t get too many questions about why the ocean is blue.

Maybe that’s because the ocean sits under the sky. So you might just assume that the oceans look blue because they reflect the sky. But if science has taught us anything, it’s that just because something seems obvious, that doesn’t mean it’s true — like in this case.

Because the real reason the ocean is blue has more to do with the fact that water is just … inherently blue. The sky is blue because of Rayleigh scattering, where bluer light bounces off air molecules better than redder light. And some of that blue does reflect off the ocean’s surface, but that’s not the main reason the ocean is blue.

The ocean is blue mostly from absorbing light, not from reflecting it. Different types of light can make water molecules vibrate different ways. When light, like from the sun, hits water, light on the redder side of the spectrum has just enough energy to get those molecules wiggling.

So water absorbs red, yellow, and green light better than blue. Most water molecules can’t do much with blue light, though, so it basically just goes right on by. A small amount of water, like in a cup, might look like it lets every color through equally well, since it’s transparent.

But that cup of water actually lets slightly more blue light through than red. And in the ocean, by the time you get to about a hundred meters deep, almost all of the red light has been absorbed, so the water is a deep blue color. Water still absorbs some blue light, though, which is why the oceans are completely dark below about a kilometer deep.

Some blue light also gets reflected back toward the surface instead of absorbed on its way down, giving the Earth its beautiful blue oceans. There are some places with water that’s a different color — it might be blue-green, for example, because of algae that reflects green light. And near mouths of big rivers, the ocean can look brown because of all the dirt and silt in the river.

But no matter what’s in it, deep water looks pretty blue, because that’s the only color of sunlight left after the other colors were absorbed. Some animals have even evolved to take advantage of this by being red instead of blue. Blue animals in deep water will reflect that blue light, so they’ll be easier for predators to find — or for prey to avoid.

But with little to no red light to reflect off of red animals, animals with red skin or scales or whatever just look black in deeper water, which makes them harder to find. So, no matter what the sky looks like today, just remember that deep water is always blue. Thanks to Patreon patrons Arraffa Piédiferro and someone who just put their name as a pair of brackets for asking this question, and thanks to all our patrons, who keep these answers coming.

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