The author is riffing on Shakespeare:

  Full fathom five thy father lies;
  Of his bones are coral made;
  Those are pearls that were his eyes;
  Nothing of him that doth fade,
  But doth suffer a sea-change
  Into something rich and strange.

http://en.wikipedia.org/wiki/Full_Fathom_Five_%28Tempest%29#...

> "unfathomable" can be interpreted literally in the analogy but also holds its normal meaning

Near as I can tell without reading the article, this sentence uses "unfathomable" with all of its meanings. Which I find nifty.

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.

The fathoming happens inside the pearl.