Darkness is the most common environmental condition on the planet because it is typical of oceans at more than 200 m of depth. Even in the epipelagos (and on land), the sunlight is, in any case, limited to a variable part of the day. The most common environmental condition on the earth, consequently, is the absence of sunlight, which conditions the lifestyles of living beings, starting with the absence of photosynthesis and living organisms based on such activity. Life in the absence of light in continental underground situations obliges organisms to evolve sensitivity alternatives to eyes (photoreceptors), which are not formed in most cases. The pelagic and benthic dominions of the ocean are largely in the dark. Thus, the presence of eyes and the perception of light in the dark part of the planet seem not to be obvious. The specialized marine fauna is, however, surprisingly equipped with eyes and the possibility to perceive light. The solution to this enigma is that sunlight is not the only light used by organisms to perceive and interact with the surrounding environment. Many organisms are able to produce light (bioluminescence) with communicative, attractive, and/or defensive purposes in the dark, deep ocean, which is perhaps the main difference from the emerged continents other than the water medium. The dark environment of submarine caves, or that of dark pelagos below the euphotic layer, has already been treated in preceding chapters, but they have to be considered together with the present chapter in the building of a knowledge of the ocean dark dominion.