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.