They Found A Mysterious Shadow in the Ocean

During World War II, the US Navy noticed a mysterious shadow on their sonar. It was too shallow to be the seafloor, but more alarmingly, it moved. Every night, it would creep closer to the ship only to slink away again during the day. Was it a hidden fleet? A moving continent, a giant monster? To find out, we have to dive deep. This zone of the ocean is called the twilight zone. Light struggles to reach down here, and the food is scarce. So every night some animals rise from this deep dark place, swim hundreds of meters to the surface where they feed safely under the cover of night and then they return to the dark during the daytime to hide. A few animals doing this wouldn't show up like this on the sonar, but billions of tons of them would. These sailors were unknowingly witnessing the world's largest migration. A global event so big that it churns the nutrients of our oceans and it protects us by quickly pulling harmful carbon back deep into the sea. But today, there are still a lot of questions about what exactly is happening here. to learn about how they're going to find out more.