underside

plural undersides

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

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

   1963 March, “The Clacton express electric multiple-units enter service”, in Modern Railways, page 173

   3 more examples

   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.

   1944 May and June, “Notes and News: Tank Engine Stability”, in Railway Magazine, page 185

   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.

   2016 January 23, Michael Emons, “Crystal Palace 1–3 Tottenham”, in BBC Sport, archived from the original on 17 Sep 2018

   Among other work done, according to the letter: CPR replaced the bottom of a deteriorating stair column base with hollow steel structural sections; conducted exploratory demolition on the underside of an eave that required “removal and reconstruction”; and removed all loose “cracked, spalled, deteriorated, and delaminated concrete” and damaged stucco from the underside of balcony eaves on 114 of the building’s 136 units, the letter said.

   2021 July 2, Eliott C. McLaughlin, Alta Spells, Francesca Giuliani-Hoffman, “Firm held off on repairs because of stability issues in months before Surfside building collapse”, in CNN, archived from the original on 22 Nov 2021