The deep ocean remains the final frontier on Earth. While we have mapped the surface of Mars, nearly 80% of our own oceans remain unmapped and unobserved. One of the most fascinating aspects of this dark world is bioluminescence—the ability of organisms to create their own light. This biological light is used for everything from attracting mates to confusing predators. Perhaps the most famous location in the deep is the Mariana Trench. Reaching a depth of nearly 11,000 meters, it is a place of extreme pressure and freezing temperatures. Exploring such a hostile environment requires cutting-edge engineering and specialized titanium hulls. The sounds of the deep are equally mysterious. Low-frequency whale calls can travel for hundreds of miles across the ocean floor, serving as a complex communication network. As we continue to develop new autonomous underwater vehicles, we are finally beginning to peel back the curtain on this hidden world.