It doesn’t matter how cagey or hyperactive the guest, anyone within eyeshot of the Deep Seas episode of Planet Earth will sit down, shut up, and watch in awe. Partly this is because of the enormous teeth on the predatory fish, partly the odd, space ship-like shapes of the jellies, but mostly it’s the light show.Deep down at the bottom of the ocean, life lights up like a fireworks display. It’s not just the depths of the abyss, however. Sandy beaches often glow by night thanks to bioluminescent bacteria, as do the skies above them with buzzing fireflies. Their strategies for creating this light are as varied as their reasons for doing so, but they all have one thing in common: though it might be beautiful to human eyes, bioluminescence is a deeply practical process.As mentioned, bioluminescence is found most commonly in and around the ocean. Whole areas of ocean water, as much as 10,000 square kilometers, can glow with the cumulative effects of bioluminescent bacteria. Deep below this “milky seas effect” fish, jellies, and even insects produce a dizzying array of colors and patterns. This is primarily true for one reason: there’s less light underwater than at the surface, making bioluminescence both more necessary and more useful.There’s no terrain to block light anywhere on the ocean floor, and since there are no surrounding objects or details to act as points of reference, moving patterns of light can be uniquely disorienting to both predators and prey. It’s no coincidence that most luminescent marine species emit light in the same narrow blue-green slice of the spectrum as sunlight filtered through sea water, keeping signaling from being too obvious from too far away.The milky seas effect can even be visible from space.Still, there are any number of reasons to create light. Fireflies use an enzyme called luciferase to create their characteristic light shows and advertise their position and virility to potential mates. On the other hand, plankton are thought to light up when disturbed so that when small fish begin to eat them their light can attract larger fish to come devour the attackers. Many jellies use complex, almost extra-terrestrial light shows to dazzle predators before going dark and darting for the safety of pitch black water. Others, like the angler-fish, use a bioluminescent body portion to lure prey close enough to eat.And, of course, some animals glow in order to see. Some are looking for general illumination but others have more specific goals, like fish that emit red light. Many deep-sea fish are reddish in color to take advantage of the fact that red light is weak and gets filtered out of sunlight before it reaches their depths. By emitting red light, these fish can render such camouflage useless. Often, a species’ eyes will be tailored specifically to its own emission spectrum, providing a nice way of communicating over long distances without alerting every animal in the vicinity.Fireflies use a well-understood chemical reaction controlled by their production of the enzyme Luciferase.Whatever their reasons for doing so, different life forms have developed broadly similar strategies for creating light. In general, the organism creates one or more chemicals, usually an enzyme that catalyzes a light-producing reaction. On land this is almost always an oxygen-dependent process, and in all cases it consumes a significant amount of the animal’s energy. Often this comes in the form of ATP or other general fuel molecules, stealing from the body’s overall energy pool to light the night, or the deep. Others make use of symbiotic relationships with luminescent bacteria, which have themselves only evolved to glow because it makes them useful to their hosts.Bioluminescence is one of the most visually amazing aspects of nature. It has inspired ghost stories about fox fire and sightings of underwater aliens, teased at the imagination of artists since the beginning of time. From fungi to octopi, it’s a major feature of life, and one we’ve only really just begun to appreciate.