brazen vessels
The brazen vessels hanging in one of these mines, which was in front of a mine of the enemy, began to ring from the strokes of their iron tools. So from this it was ascertained where the enemy, pushing their mines, thought to enter. The line being thus found out, he prepared kettles of hot water, pitch, human excrement, and sand heated to a glow. Then, at night, he pierced a number of holes, and pouring the mixture suddenly through them, killed all the enemy who were engaged in this work. (Vitruvius, 15BCE)