godan verb (-mu), intransitive verb
sink into; sink down
Physical sinking into a surface or substance, often with a sense of depth or being enveloped.
ぬかるみに足が沈み込んだ。
My foot sank into the mud.
ソファに体が沈み込むほど柔らかい。
The sofa is so soft that your body sinks into it.
godan verb (-mu), intransitive verb
get depressed; sink into depression
Metaphorical use describing a person's mood sinking deeply into a depressed state.
失恋してから、彼は沈み込んでしまった。
After the heartbreak, he sank into depression.