Twain studied the Mississippi, learning its landmarks, how to navigate its currents effectively, and how to "read the river" and its constantly shifting channels, reefs, submerged snags and rocks that would "tear the life out of the strongest vessel that ever floated". It was more than two years before he received his pilot's license. Piloting gave him his pen name, Mark Twain, from "mark twain", the leadsman's cry for a measured river depth of two fathoms (12 feet), which was safe water for a steamboat.