False bottom

A false bottom is an internal partition in a container, typically at the bottom of a drawer, suitcase or similar item, enabling the concealment of objects from a cursory examination of the item's contents. A false bottom may be present if the internal dimensions of a container do not match its external dimensions after taking into account the thicknesses of constituent materials and structures used to build the object.

Drawer with a false bottom (actually a false back in this case)

The term false bottom can also refer to the deep scattering layer in the ocean, a phenomenon where a layer of marine organisms deep in the ocean can be mistaken by sonar for the seabed.[1]