For this to work beautifully, we would fathom pearls when finding or collecting them. afaik only depths of water can be the literal object of the verb to fathom. It doesn't quite succeed, here. Which is likely why Pynchon is celebrated and the reviewer is reviewing him, rather than the reverse.
Near as I can tell without reading the article, this sentence uses "unfathomable" with all of its meanings. Which I find nifty.