Something fathomless is so deep or great that you can’t measure it, or so mysterious that nobody can understand it. If you were an ant, you’d probably think the kitchen sink was fathomless — and also the whole business of washing dishes!
Fathomless means about the same as the more common word unfathomable. You might choose fathomless in a poem, or in any piece of writing where you want to sound a little more elegant or literary — or use three syllables instead of five! In your reading you might come across expressions like "fathomless oceans" or "the fathomless universe." Figuratively, knowledge, love, evil, and wealth are often described as fathomless.