subcrust
/ˈsʌbkrʌst/adjective
Attributive Lying or taking place beneath a crust, especially ( Geology ) beneath the crust of the earth.
noun
A layer that underlies a crust; specifically (a) Geology a (semi-molten) layer of rock beneath the earth's crust; (b) a supporting layer beneath the upper surface of a road.
Origin
Late 19th century. From sub- + crust.