substratum
noun (specialist)a layer of something, especially rock or soil, that is below another layer （尤指岩石或土壤的）下卧层；底土层 a substratum of clay 粘土底层 the substrata of society 社会底层
Word Originmid 17th cent.: modern Latin, neuter past participle (used as a noun) of Latin substernere, from sub- ‘below’ + sternere ‘strew’. Compare with stratum.