This Radiolab interview podcast explores the extreme blackness of certain deep-sea fish, specifically anglerfish and dragonfish. The discussion begins with the host's experience of darkness at sea, leading into an interview with biologist Sanka Johnson about his research on these unusually dark creatures. Johnson details his research process, which involved comparing the fish's blackness to that of butterfly wings and ultimately using mathematical models developed for studying the moon's surface to understand the fish's unique structure. The research revealed that the fish achieve their extreme blackness through a dense concentration of melanosomes, tiny light-absorbing structures within their skin cells, arranged in an optimal configuration. This research has implications for engineering applications requiring ultra-black materials.