The "six" in the expression comes from the six feet that make up a fathom which is a little less than two meters.

VOA: special.2009.05.24

The "six" in the expression comes from the six feet that make up a fathom -- which is a little less than two meters.

VOA: special.2011.03.06

Down two and a half. Kicking up some dust. Thirty feet.

VOA: special.2009.07.15

Two standing ordersin this platoon... 1... take care of your feet, 2... try not to doanything stupid, Like gettingyourself killed.

VOA: standard.other