On a summer’s night, there’s nothing more magic than watching the soft glow of fireflies switching on and off. Few other life forms on land can light up the night, but in the dark depths of the oceans, it’s a different story: nearly 90% of all species shine from within. Whether it’s to scare off predators, fish for prey, or lure a mate, the language of light is everywhere in the ocean depths, and scientists are finally starting to decode it. National Geographic take a dazzling dive to this hidden undersea world where most creatures flash, sparkle, shimmer, or simply glow. Join deep-sea scientists who investigate these stunning displays and discover surprising ways to harness nature’s light—from tracking cancer cells to detecting pollution, lighting up cities, and even illuminating the inner workings of our brains.