sapping

thumb|325px|Union troops of the 1st New York Engineers digging a sap with a sap roller on [[Morris Island, 1863]]

Sapping is a term used in siege operations to describe the digging of a covered trench (a "sap") to approach a besieged place without danger from the enemy's fire. The purpose of the sap is usually to advance a besieging army's position towards an attacked fortification. It is excavated by specialised military units, whose members are often called sappers.