Unsolved Mystery 4: The Abyss Inside Earth

How many fish live in the ocean’s mysterious “twilight zone”?

As you dive deeper into the ocean, less and less sunlight shines through. About 200 meters beneath the surface, you reach an area called the mesopelagic, or the “twilight zone.” Sunlight fades almost completely out of view, and our knowledge about these dark depths fades too.

Yet this region of the ocean is extremely important. It’s possible — but not certain — that more fish are living in the twilight zone than the rest of the ocean combined, and these creatures of the dark ocean play a large role in regulating the climate.