Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
fathom [ˈfæðəm] n (NAUT) фа́том, морска́я са́жень f
vt (understand) (also: fathom out)постига́ть (пости́чь*perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: