The Collaborative International Dictionary of English v.0.48:

Underside \Un"der*side`\, n.
   The lower or lowest side of anything. --Paley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underside
    n 1: the lower side of anything [syn: bottom, underside,
         undersurface]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "underside":
   bed, bedrock, belly, bottom, bottom side, breech, buttocks,
   downside, fundament, hardpan, lower side, lowest layer,
   lowest level, nether side, nethermost level, rock bottom, sole,
   substratum, underbelly, underlayer, underneath