The least glamorous but often the most effective way of breaching the walls was by mining, that is, tunneling under the wall (Document 27). Obviously, mining was used where a castle was not built on solid rock or surrounded by water. The miners propped up the tunnel with timber as they dug so that, when the wood was burned, the unsupported wall came crashing down. The miners might tie kindling to pigs, set the poor beasts alight, and drive them into the tunnel to ignite the timbers. The fat of the burning pigs increased the intensity of the fire. To defend against mining, the castle occupants excavated their own tunnel, a technique known as countermining. They could either break through to the rival tunnel and engage in underground combat, or they could light fires and drive the smoke into their opponent’s tunnel, making work impossible.