a unit for measuring the depth of water, equal to 1.8 metres or 6 feet

to discover the meaning of something

For years people have been trying to fathom (out) the mysteries of the whale's song. 多年来人们一直试图弄清鲸鱼歌声之谜。

to understand someone or why someone acts as they do

I can't fathom her at all. 我一点儿也搞不懂她。