the layer of soil underneath the soil on the surface; a substratum or subsoil
understratum in American English
(ˈundərˌstreitəm, -ˌstrætəm)
nounWord forms: plural-strata (-ˌstreitə, -ˌstrætə) or -stratums
a substratum
Word origin
[1725–35; under- + stratum]This word is first recorded in the period 1725–35. Other words that entered Englishat around the same time include: amorphous, collapse, copyright, snowflake, viburnumunder- is a prefixal use of under, as to indicate place or situation below or beneath (underbrush; undertow); lower in grade or dignity (undersheriff; understudy); of lesser degree, extent, or amount (undersized); or insufficiency (underfeed)