substratum
noun /ˈsʌbstrɑːtəm/
  /ˈsʌbstreɪtəm/
(plural substrata 
 (specialist) /ˈsʌbstrɑːtə/
  /ˈsʌbstreɪtə/
)- a layer of something, especially rock or soil, that is below another layer- a substratum of clay
- the substrata of society
 Word Originmid 17th cent.: modern Latin, neuter past participle (used as a noun) of Latin substernere, from sub- ‘below’ + sternere ‘strew’. Compare with stratum.