A fathom is an old measure of how deep water is — about as long as a person’s two arms stretched out. So fathomless first meant water so deep you cannot measure it. People now use it for feelings and ideas you cannot find the bottom of: fathomless sadness, fathomless space, a fathomless mystery. Though actually, in everyday talk unfathomable is more common when you mean “I cannot understand this.” Fathomless sounds a little like a book: you will see it in careful writing more than hear it in the shop. It still works for the sea and for the mind.

深不可測的（海、宇宙、或心思都說得通）