Understratum refers to the layer or level beneath the surface or top layer. Some synonyms for understratum include substratum, foundation, base, underlying layer, groundwork, and substructure. These terms are often used interchangeably to refer to the layer upon which another layer is built or exists. For example, in construction, the foundation of a building serves as the understratum upon which the rest of the building is erected. Similarly, the substratum of soil supports the growth of plants and vegetation above it. In science, the underlying layer of rock provides clues about the geological history of an area. Regardless of the context, understratum and its synonyms refer to the foundation upon which something else rests.