potonuti

1. to sink
   Brod je potonuo za nekoliko minuta. — The ship sank in a few minutes.
   Kamen je odmah potonuo. — The stone sank right away.
2. (of a person) to become despondent; to lose heart
   Posle tog poraza potpuno je potonuo. — After that defeat he completely lost heart.

The verb is intransitive; to sink something is potopiti.