名词（土壤的）底土，心土
1. 底土;心土;下层土
The subsoil is a layer of earth that is just below the surface soil but above hard rock.
e.g. ...the chalk subsoil on the site.
那个工地的白垩底土
1. The hydrates were found in the permanently frozen subsoil to the south of Qilian Mountain in Qinghai province.
noun
1. the layer of soil between the topsoil and bedrock
Synonym: undersoil