We may be able to plant the Union Jack on the summit of Mount Everest, we may drag our sledges to the South Pole, and we may, some day, be able to travel with ease and safety in the Great Sahara; but we cannot conceive that it will ever be possible for us to invent a diving-bell that will take a party of explorers to a depth of three and a half miles of water. We may complete our survey of the ocean beds, we may analyse the bottom muds and name and classify the animals that compose their fauna, but there are many things that must remain merely matters of conjecture. We shall never know, for example, with any degree of certainty, how _Bathypterois_ uses its long feeler-like pectoral fins, nor the meaning of the fierce armature of _Lithodes ferox_; why the deep-sea Crustacea are so uniformly coloured red, or the intensity of the phosphorescent light emitted by the Alcyonaria and Echinoderms. These and many others are and must remain among the mysteries of the abyss. Our present-day knowledge of the physical conditions of the bottom of the deep sea and the animals that dwell there is by no means inconsiderable. It may be found in the reports of the scientific expeditions fitted out by the English, French, German, Italian, Norwegian, and American Governments, in numerous volumes devoted to this kind of work, and in memoirs and notes scattered through the English and foreign scientific journals. It is the object of this little book to bring together in a small compass some of the more important facts and considerations that may be found in this great mass of literature, and to present them in such a form that they may be of interest to those who do not possess a specialist’s knowledge of genera and species. When it was found that animals can and do live even at the greatest depths of the ocean, the interest of naturalists was concentrated on the solution of the following problems. Firstly, do the animals constituting the fauna of the abyss exhibit any striking and constant modification in correlation with the physical conditions of their strange habitat? And, secondly, from what source was the fauna of the abyss derived? Was it derived from the shallow shore waters, or from the surface of the sea? Is it of very ancient origin, or the result of, comparatively speaking, recent immigrations? These questions cannot be answered in a few lines. Any views that may be put forward regarding them require the support of a vast array of facts and figures; but as the limits of this little book would not permit of my giving these, I have endeavoured to select a few only of those which bear most directly upon the points at issue. To overburden my work with the names of genera or the lists of species would not, it seemed to me, either clear the issues or interest the general reader. These may be found in the ‘Challenger’ monographs, and other books dealing with the subject.