Verb derived from noun 窪 / 凹 (kubo, “an indentation, a depression”) + verbalizing suffix ぶ (bu, “becoming like [noun]”), with suffix ぶ (bu) mutating to む (mu) due to the preceding syllable ぼ (bo).
窪む (intransitive, godan conjugation, hiragana くぼむ, rōmaji kubomu)
The transitive form is 窪める / 凹める (kubomeru). Consequently, the potential form is the same as the passive form, くぼまれる (kubomareru), instead of くぼめる (kubomeru) as shown incorrectly in the table above.
The spelling 窪む is used more often for くぼむ, because the alternate spelling 凹む can also be read as へこむ.