fundo

Fundo is the inside bottom, the surface you reach when you go all the way down: the bed of a river, the base of a glass, the far end of a courtyard. For the underside of an object seen from outside, say la malsupra flanko. The figurative use survives intact from the literal one, so en la fundo de la koro is idiomatic and unremarkable. Profunda is the adjective for deep, and funde as an adverb means thoroughly, right down to the bottom of the matter.