Military mining is an ancient procedure that has often been used in the reduction of fortresses during siege warfare. During the First World War (1914-1918), the Western Front that crossed western Europe was effectively a linear fortress. Military mining was employed by all protagonists and involved the construction of dugouts - underground shelters for housing men, as well as the construction of subways for movement of men and materiel, and mined tunnels for offensive action. All these excavations connected with trenches: open ditches up to 2 m deep. As the armies left the field of battle at the end of the war, trenches were most often filled in upon the return to agriculture, but tunnels and dugouts, often with small entrances, were most often simply blocked off leaving open subterranean voids. This is a particular problem in Flanders, where tunnels and dugouts were cut through a range of weak sediments, usually supported by timber. Advancing age, and changes in depth to reach water saturated ground are some of the features that have led to incidences of structural damage as a result of subsidence. Four case studies in Flanders demonstrate the need for detailed surveys and hazard mapping in the region. Rich archives existing in Europe and elsewhere demonstrate that the construction of historical engineering maps as an aid to planning is feasible.