subsurface
adj. C1 Advanced US //ˈsəbˌsɝfəs// sub·sur·face
adj. describing something that is hidden or located below the surface of the ground or water.
adj. situated or occurring beneath the surface of the earth or a liquid. Often used in geological or mathematical contexts to describe structures or layers that are not visible from above.
The subsurface layers of the soil are very dry.
Geologists use seismic waves to map the subsurface structure of the earth to find oil and gas reserves.
In the study of fluid dynamics, the subsurface flow of water through porous rock is critical for understanding how contaminants might spread across an aquifer.