The deep sea, which gets darker with increasing depth until no more sunlight penetrates at about a kilometer depth, and ever colder closer to the bottom of the ocean, covers most of the planet and is thus by far the largest habitat on earth, yet has been explored less than space, so most scientific expeditions, at depths requiring modern submarine technology, discover at least one new species, or even whole new branches of submarine life. Like everywhere else, evolution has over millions of years produced several amazing adaptations to even the most extreme conditions, here especially to the lack of sunlight, with its problems for procreation, searching food and fleeing hunters, such as photophore cells which produce specific light types, mimic rare light penetrating from above, etcetera.