A nautical expression indicating a water depth of 6 fathoms (36 feet, 10.97 metres) as measured by a sounding line; "deep six" acquired its idiomatic definition because something thrown overboard at or greater than this depth would be difficult, if not impossible, to recover. Marks on a sounding line were traditionally placed at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. The "leadsman" called out the depth as he read it off the line. If the depth was at a mark he would call "by the mark" followed by the number, if the depth was between two marks, he would call "by the deep" followed by the estimated number. Six fathoms would therefore be reported as "by the deep six".