Sometimes the best way to answer a question like "How many animals on the bottom of the ocean glow?" is to just go down there and poke some sea creatures with a robot arm. That's how researchers found out that the pitch-black seafloor in the Bahamas is alive with bioluminescence. They also found glowing currents full of plankton, a crustacean with the world's slowest vision, and creatures that vomit light when provoked.