substratum
/ˌsʌbˈstrɑːtəm/noun
(pl.substrata /-tə/)
1- an underlying layer or substance, in particular a layer of rock or soil beneath the surface of the ground底层, 下层（尤指地表下的一层岩石或土）。
1.1
- a foundation or basis of something基础; 根据：
there is a broad substratum of truth in it.
这其中有广泛的事实做基础。
词源
mid 17th cent.: modern Latin, neuter past participle (used as a noun) of Latin substernere, from sub- 'below' + sternere 'strew'. Compare with STRATUM.