utonuti

1. (into water, mud, snow) to sink
   Čamac je polako utonuo u reku. — The boat slowly sank into the river.
   Noge su mi utonule u blato do kolena. — My feet sank knee-deep into the mud.
2. (into an armchair, sofa) to sink into
   Posle posla je utonuo u fotelju i zaspao. — After work he sank into the armchair and fell asleep.
3. (into sleep, thought, silence) to sink into; to fall into
   Dete je brzo utonulo u san. — The child quickly fell into a deep sleep.
   Ceo grad je utonuo u mrak. — The whole city sank into darkness.

Usually used with u + accusative: utonuti u san, u fotelju.