The Collaborative International Dictionary of English v.0.48:

Underlayer \Un"der*lay`er\, n.
   1. One who, or that which, underlays or is underlaid; a lower
      layer.
      [1913 Webster]

   2. (Mining) A perpendicular shaft sunk to cut the lode at any
      required depth. --Weale.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "underlayer":
   band, bed, bedding, bedrock, belly, belt, bottom, bottom side,
   breech, buttocks, couche, course, deck, downside, floor, fundament,
   gallery, hardpan, layer, ledge, level, lower side, lowest layer,
   lowest level, measures, nether side, nethermost level, overlayer,
   overstory, rock bottom, seam, shelf, stage, step, story, stratum,
   substratum, superstratum, thickness, tier, topsoil, underbelly,
   underneath, underside, understory, understratum, zone