A large part of the planet’s organisms thrive in conditions very different from those that we consider “normal”. If the opposite of “normal” is “extreme”, the deep ocean requires evolutionary adaptations to extreme conditions such as high pressure, darkness, and the absence of any substratum where the organisms can rely upon and orient themselves. Many terrestrial-derived taxa invaded the ocean habitat and evolved adaptations for this new lifestyle. Although they are conditioned by some terrestrial distinguishing marks (such as the breathing of air), most of them are typically oceanic, and some species are able to perform regular excursions in the deep. These species (belonging to Reptilia, Aves, and Mammalia) have no gills to be used in extracting oxygen dissolved in the water; thus, they have evolved the possibility of storing oxygen after inhalation from the air to allow deep and/or long dives under physiological conditions, which has the name apnoea. Notwithstanding the physiological link with the air, such organisms evolved the ability to go into deep, dark oceans, regularly, even at more than 1000 m below the sea surface. The solutions adopted to perform long and deep dives are the arguments of the present chapter.