IPA: /ˈʌndərˌsɜːrfɪs/
KK: /ˈʌndərˌsɜrfɪs/
Located or occurring beneath the top layer of something.
The undersurface of the leaf is often a different color than the top side.
The bottom or lower surface of something, often not visible from the top.
The undersurface of the leaf is usually a different color than the top side.
Undersurface → It is formed from "under-" (meaning beneath) and "surface" (from Latin "superficies", meaning the outer face or appearance). The word "undersurface" refers to the surface that is beneath the top layer or exterior.
Think of something that is 'beneath' ('under-') the 'outer layer' ('surface') — that's what undersurface means.