Undersoils are the underlying layer of soil located beneath the topsoil layer. There are several synonyms that can be used to describe it. For instance, subsoil is the layer of soil that lies below the topsoil and above the parent material, which is the underlying rock or sediment. The term substratum is also often used to describe the undersoil. Additionally, regolith refers to the layer of loose material covering solid rock, and the term saprolite refers to extensively weathered and decomposed rock that forms soft, crumbly soil. In summary, these synonyms provide alternative ways to refer to the undersoils, depending on the context in which they are used.