How many?

Feet

The foot is a unit of length in the British imperial and United States customary systems of measurement. Since the International Yard and Pound Agreement of 1959, one foot is defined as 0.3048 meters exactly. In both customary and imperial units, one foot comprises 12 inches and one yard comprises three feet. The foot is taken from the average length of a mans foot, but in older times was just uner 12 inches.

Fathoms

A fathom is a unit of length in the imperial and the U.S. customary systems equal to 72 inches (1,828.8000 mm), used especially for measuring the depth of water. The Atlantic Ocean at its deepest point is believed to be 4,580 fathoms deep.