n. 25B6; verb
Charlie tried to fathom her expression : UNDERSTAND, comprehend, work out, make sense of, grasp, divine, puzzle out, get to the bottom of; interpret, decipher, decode; informal make head or tail of, tumble to, crack; Brit. informal twig, suss (out), savvy.
fathoming the ocean : MEASURE THE DEPTH OF, sound, plumb.