substratum
noun
[ sʌbˈstrɑːtəm ]
• an underlying layer or substance, in particular a layer of rock or soil beneath the surface of the ground.
• "the plant will grow very rapidly and send out runners above the substratum"
Origin:
mid 17th century: modern Latin, neuter past participle (used as a noun) of Latin substernere, from sub- ‘below’ + sternere ‘strew’. Compare with stratum.
