Although the oceans cover more than 70% of the planet’s surface, humans are largely unfamiliar with them. Fisheries are the main human activity deeply interacting with the sea. This activity, however, has only recently developed capture devices and/or fisheries reaching maximum depths close to 1000 m below the sea surface (such a layer includes 25% of the average depth of the ocean water column). Such a deep-sea frequentation, in any case, is mainly devoted to collecting resources from the sea bottom, thus ignoring what floats in the above standing water column. Mineral searching and extraction devices allow work in deeper situations than the lower fishery limit, but these activities also interact mainly with the bottom, disregarding the water column. The life in the water column proposes different problems to living organisms (such as sinking avoidance, the absence of shelters, and the immensity of the volume to cross) coupled with those commonly affecting the rest of marine life (darkness, low temperature, high pressure) below the tiny euphotic layer at the surface. Pelagos is the aquatic, fluid world where living organisms have to spend their lives without being conditioned or in some way affected by a solid substrate. The largest organisms obtain such a condition by living far from the coastline, but the smallest ones, such as unicellular organisms, can be considered pelagic even if they thrive in coastal or confined water areas, just because they stay suspended in the water throughout their (short) lives. The pelagic lifestyle, as a consequence, is even a water drop for a bacterium, and it is typical of every part of the ocean extension, with a partitioning proposed either in a vertical or horizontal sense on the basis of shared physical-chemical properties of the water masses and of the biological assemblages. Five horizontal layers are recognized from surface to bottom with levels of knowledge decreasing with the depth. Additionally, due to this lack of knowledge, a simplification of biogeography is observable, with 4 realms and 37 bioprovinces only for the surface pelagos (epipelagos), with the deeper layers being more extended and simpler. For example, the biological assemblages of the Mediterranean Sea allow the identification of 32 different surface areas, with the eastern part “disturbed” by the entrance of Red Sea fauna, thanks to the recent (1869) opening of the Suez Canal (the so-called Lessepsian migration).