Old English under licgan "to be subordinate to, to submit to;" see under + lie (v.2). Meaning "to lie under or beneath" is attested from c. 1600; figurative sense of "to be the basis of" is attested from 1852 (implied in underlying). Similar formation in Old High German untarliggan; German unterliegen.
Underground Railroad
undergrowth
underhand
underhanded
underlay
underlie
underline
underling
underlying
undermine
undern