Several aerial tramways, thousands of feet long, connected various adits and mills. These included at least 52 bell pits, some with deeper shafts and 16 adits. Predecessors of modern tunnels were adits to transport water for irrigation or drinking, and sewerage. The mines extended nearly through four adits. Thus the area was stabilised by buttressing and inserting drainage adits. From the canal tunnel, thirteen adits were driven to facilitate excavating the railway tunnel. Such workings were small-scale and near the surface, using adits and bell pits. The mountains are riddled with adits and holes. The use of adits for the extraction of ore is generally called drift mining. Rare Neolithic pictures are engraved on the walls of some of these adits. The coal was worked through a drift mine with additional adits for ventilation and drainage. The same adits also served to drain the workings. The development work consisted of two adits and a shaft deep. Drainage adits interlink with one another, including the canal tunnel, into which water is discharged. All adits to the mine are now blocked.