He was so submerged in his music composition that he lost track of time.

submerged 是动词 submerge 的过去分词形式。submerge 由前缀 sub-（表示"在...下面"）和词根 merge（"浸入、合并"）组成，字面意思是"浸入下方"。在这里用作比喻意义，表示"完全沉浸于某事"的状态。