Bioluminescence

Records of "glowing" animals date back to Ancient Rome and Greece, and bioluminescence has long interested scientists and intellectuals. Aristotle thoroughly investigated the phenomenon and was the first to postulate—accurately—that these animals don't use heat to generate light. Since then, we've discovered numerous species, both on land and in the water, who possess this skill, each with a distinct method of employing it.

For land creatures, it's fairly well understood, but in the oceans, especially in the deep sea, the phenomena are far more pervasive and intricate. We discover that there may simply be an entire language and system of communication in a region absolutely devoid of light that we simply don't comprehend when we delve further and discover more species that can make light in a variety of ways. A research found that 76% of marine species possess this ability, albeit it is unknown how most of them use it.