"Cast the lead" is a phrase commonly used in boating. It refers to the act of dropping a lead weight attached to a line over the side of a boat in order to measure the depth of the water below. While the phrase itself has been in use for centuries, there are several synonyms that can be used to describe this activity. These include "sounding," "taking a depth measurement," "checking the depth," or even "throwing the line." No matter how one chooses to describe it, cast the lead remains an essential and time-honored way to navigate safely through open waters.