Fathom was upset and rose to the surface. A "shot", one of the forged lengths of chain joined by shackles to form an anchor cable, was usually 15 fathoms long (). A "shackle", a length of cable or chain equal to 12.5 fathoms or 75 feet. In 1949, the British navy redefined the shackle to be 15 fathoms. In Finland, fathom ("syli") is sometimes, albeit seldom, used as a maritime unit, of a nautical mile and of cable length. It is customary, when burying the dead, to bury the coffin at least to a depth of a fathom, or "six feet
Fathom Castle was being built c. 1180-1200, it most probably was a fortified structure occupied by a knight in the service of DeLacy. There is definite mention of Sir Robert Marmion being at a prest of knights before King John in Dublin in 1210. It would appear that this Sir Robert was the knight responsible for Fidun/Fathom Castle. He entered Ireland with Strongbow and other Normans in 1169-1172 as documented, and there is mention of other Marmions, presumably his descendants being 'of Fidun', i.e. Sir Gilbert in 1290 and a William as late as 1305. With the Gaelic resurgence, the 'Fidun' location