A few years ago, a team of scientists took an expensive robot, attached it to a buoy floating off the coast of Hawaii, and left it there. From the outside, it would have looked like an elaborate garbage can. Inside, it was busy. As it bobbed and flowed with the currents, it sucked in some of the surrounding water and passed it through a small circular filter. It added preservative to the filter, moved it to one side, and put a new one in its place. It did this every two hours.

After three days, the scientists came back for it. That was how they took the ocean’s pulse.

The open ocean is full of life. Even when you can’t see