Verb (5-dan, む, intransitive)
1. to sink; to go under; to submerge
このままでは船が沈んでしまうかもしれない。If things continue like this, the ship might sink.
2. to go down (e.g. sun); to set; to descend
もうほとんど日は沈んでるしね。The sun has almost set already, you know.
3. to feel depressed; to become subdued; to become somber
どうも今日は沈んでばかりだ。I've just been feeling down all day today.