under surface (noun) undersurface: bottom
Sinonime: base, basement,
basis,
bed, bedrock,
belly, deepest part, depths,
floor,
foot, footing,
ground, groundwork, lowest part,
nadir, nether portion, pedestal, pediment,
rest,
seat,
sole, substratum, substructure,
support, terra firma, underbelly, underneath, underside