An autonomous reef monitoring structure (ARMS) in the mesophotic zone. | © Luiz Rocha PALAU, Micronesia — On a bright, sunny day last May, I stood on a boat surrounded by the turquoise waters of the island nation of Palau. I was there in the western Pacific watching scuba divers as they prepared to bring up evidence of a mysterious aquatic world.  Palau is famous for its spectacular and rich coral reefs. But these divers were planning to descend past those sun-lit marine communities and down into a lesser-known twilight zone known as the “mesophotic coral ecosystems.”  Mesophotic es...