sub·stra·tum 
/ˈsʌbˌstreɪtəm/ noun
plural sub·stra·ta /ˈsʌbˌstreɪtəm/ noun /-ˌstreɪtə/ [count] : a layer of something (such as soil or rock) that is under another layer
▪ drilled into the bedrock substratum — often used figuratively ▪ the city's social substrata
▪ drilled into the bedrock substratum — often used figuratively ▪ the city's social substrata
