underrating

(noun) an estimation that is too low; an estimate that is less than the true or actual value. Synonyms: underestimate, underestimation, underreckoning.

(verb) make too low an estimate of. Synonyms: underestimate, underrate. Examples: "He underestimated the work that went into the renovation." "Don't underestimate the danger of such a raft trip on this river."