The movements of can be detected by depth recorders, and this layer is known as the deep scattering layer.

Large numbers of small organisms

Explanation:

Deep-scattering layer is the layer that consists of many organisms living in the ocean. These layers are usually found in the night. They tend to sink down during the day. Sonars detected the layer of the marine organisms. Such layers of living organisms in the ocean is termed as deep scattering layer. Prior to the discovery of this layer, the layer was mistakenly considered as the seabed.