Drainage
Most adits are designed to slope slightly upwards from the entrance so that water will flow freely out of the mine. Mines that have adits can be at least partly drained of water by gravity alone or power-assisted gravity. The depth to which a mine can be drained by gravity alone is defined by the deepest open adit which is known as the "drainage adit". Workings above this level (known as "above adit") will remain unflooded as long as the adit does not become blocked. All mine workings below both the drainage adit ("below adit") and the water table will flood unless mechanical means are used for drainage. Until the invention of the steam engine this was the main restriction on deep mining. Of course, adits are still useful for deeper mines because the water only needs to be raised to the drainage adit rather than to the surface.
Because of the great reduction in ongoing costs that a drainage adit can provide, they have sometimes been driven for great distances just for this purpose, one such being the Milwr tunnel in North Wales, which is about ten miles (16 km) long. Another example is the Great County Adit in Cornwall, a 40-mile (64 km)-long network of adits that used to drain the whole Gwennap mining area. A side benefit of driving such extensive adits is that previously unknown ore-bodies can be discovered, helping finance the enormous cost.
Adits were in use in Cornwall well before 1500 and were particularly important to the tin and copper mines in Cornwall and Devon because the ore-bearing veins here are near-vertical and act as channels through which water can easily seep. There are many reports of torrents of warm or hot water continuously pouring into the lower levels of Cornish mines.
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