a unit of length equal to six feet (1.8 metres), chiefly used in reference to the depth of water

sonar says that we're in eighteen fathoms

understand (a difficult problem or an enigmatic person) after much thought

the locals could not fathom out the reason behind his new-found prosperity

he couldn't fathom why she was being so anxious

measure the depth of (water)

an attempt to fathom the ocean