How to detect the enemy when they are underground

FROM THE collapsing of city walls during medieval sieges, to the laying of giant mines filled with explosives under enemy trenches during the first world war, to the supply networks of the Ho Chi Minh trail during the war in Vietnam, to the Tora Bora cave complex, used first by the Mujahideen to oppose the Soviet invasion of Afghanistan and then by the Taliban to oppose the American invasion, tunnel-digging in times of conflict has a long history. These days, secret tunnels are used to move weapons and people between Gaza and Egypt, and by Kurdish militia operating on the frontier between Syria and Turkey. But the same principle applies. What happens underground is hard for the enemy to observe. Digging for victory is therefore often a good idea.