Many reefs around islands in the Pacific Ocean are in near-pristine condition. In these ecosystems the biomass of apex predators, like this shark, is greater than that of the smaller predators and herbivores that they eat.

After the final equipment check, you are ready to begin your mission into an incredible world. Holding your facemask, you fall backward and enter the water upside down. For a moment, you lose all sense of balance and direction. Which way is up? Before your orientation returns, you have already floated to the surface. There, buoyed by an inflated dive vest, you await the arrival of the remaining divers. When your team is assembled, you are ready to descend. Releasing the air in your vest, you begin to drop below the wave-tossed surface. Instantly, a beautiful and calm world appears before your eyes. For some people, this is the experience of a lifetime. For researcher Dr. Enric Sala, who studies the function of marine ecosystems around the world, it is just another day at work.

Today, Enric’s dive is in the shallow lagoon of Kingman Reef, part of the Line Islands in the Pacific Ocean. His living laboratory is about 1600 km (994 mi) south-southwest of Hawai‘i. Here, the ecosystem remains mostly unaffected by the activities of people. Without warning, a shark darts out of the shadows and snatches a reef fish into its jaws. One of the researchers records observations of all the reef predators they encounter on this dive. At the study site, divers swim along a straight transect line to count and identify the local organisms. Back in the lab, Enric will use these data to understand the relationships between the diverse organisms and the transfer of matter and energy in this tropical coral reef ecosystem.