The area of the deep sea has not yet been sufficiently explored, due to the specificity of that environment and difficult conditions. It is very deep, with little to no lighting, making it difficult to explore its features. In addition to making research difficult for the human hand, it also makes life difficult for the fauna of the deep sea. The animals that reside there live in extreme conditions, and are exposed to sudden changes in temperature, high pressure and other unfavorable conditions, which make it very difficult for them to live in the deep sea. Precisely because of this, they had to adapt to such a way of the environment leading many to develop the ability to illuminate their surroundings via their organs. The fish and other species that live there generally look scarier than the species that live in the shallower parts of the sea, but this very appearance is responsible for their successful survival in extremely poor conditions. Therefore, the aim of this paper is to describe the deep-sea environment and the deep-sea fauna, that is, some of the species that live there with their features and biological characteristics.