underside

noun

1. The side that is below or underneath, the bottom.

   “A correspondent suggested that the superior riding qualities of the latter were due to the absence of piston-valve chests above the cylinders, which made it possible to bring the underside of the water-tanks to a lower level than that of the "Rivers", so minimising the unsteadying effect of water surging in the tanks.”

   “[...] and the neat reading lamps on the underside of the luggage racks, under passenger control, are an attractive feature of the first class compartments.”

   “The Eagles then hit the woodwork twice in a matter of seconds as Scott Dann headed Jason Puncheon's free-kick against the underside of the bar, before substitute Mile Jedinak also hit the crossbar with a shot on the turn.”