Leonardo da Vinci designed a countermine to protect a fortress from enemy tunnels.
In siege warfare, attackers could dig mines beneath walls to weaken foundations or place explosives. A countermine was a defensive tunnel dug from inside a fortification to intercept, collapse, or attack those underground passages. Leonardo’s notebooks include plans for detecting and dealing with such subterranean threats.
His military engineering studies covered fortifications, artillery, bridges, and water management. The countermine concept shows that Leonardo was applying observation and geometry to problems below ground as well as above it. Defenders needed to understand the direction and depth of enemy digging while avoiding the collapse of their own works.
A countermine is not the same as a moat, which is an open trench around a fortification, or a land mine, which is an explosive device. The word mine in this context means a tunnel used in siege operations.