When sails were canvas and square and everywhere along the horizon, depth measurements were taken with a weighted length of line that trailed off into the sea behind: sounding.
The increment of choice is a fathom -- the distance between two outstretched arms. Averaged, somewhere around six feet. The sounding line is scientifically dangled over the ship railings or gunwales (or, preferring the sound, 'gunnels' ). A sailor carries on, counting off the chain of fathoms in knots he ties at each outstretching of his arms. Like a warped pearl or a clenched fist, the plummet clings to the end of the line and tugs the line towards the sea floor with all the weight and density of lead. As the end dips into the salt-cool waves the line uncoils. Slack runs over the edge until the deck is empty and clean and smells faintly of linseed oil.
The known distance of the sea floor, any depth ever written down, all recorded soundings, used to be bounded by a sailor's ability to spin out this nautical thread.
What we know as unfathomable is simply so because the line slipping over the gunwales and into the sea is too short.