page_title: Adit
section_title: Drainage
hierarchical_section_title: Adit / Drainage
caption_attribution_description: Entrance to the Two Brothers adit at Eylesbarrow mine on Dartmoor in South Devon, UK.
context_page_description: An adit is an entrance to an underground mine which is horizontal or nearly horizontal, by which the mine can be entered, drained of water, ventilated, and minerals extracted at the lowest convenient level. Adits are also used to explore for mineral veins.
context_section_description: 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". The term mine drainage tunnel is also common, at least in the United States.  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. Adits are useful for deeper mines, as 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 for this purpose. One example is the Milwr tunnel in North Wales, which is about ten miles (16 km) long. Other examples are the Great County Adit in Cornwall, a 40-mile (65 km)-long network of adits that used to drain the whole Gwennap mining area, and the 3.9 mile (6.2 km) Sutro Tunnel at the Comstock Lode in Virginia City, Nevada. A side benefit of driving such extensive adits is that previously unknown ore-bodies can be discovered, helping finance the enormous cost.
  Adits were used in Cornwall before 1500, and were important to the tin and copper mines in Cornwall and Devon because the ore-bearing veins are nearly vertical, thus acting as ingress channels for water.
caption: A small, almost hidden adit entrance at Eylesbarrow mine, Dartmoor, Devon
exif: {"Image ImageDescription": "SONY DSC", "Image Make": "SONY", "Image Model": "DSLR-A350", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "DSLR-A350 v1.00", "Image DateTime": "2009:04:02 11:58:43", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "444", "Image CustomRendered": "Normal", "Image ExposureMode": "Auto Exposure", "Image WhiteBalance": "Manual", "Image FocalLengthIn35mmFilm": "27", "Image SceneCaptureType": "Standard", "Image Contrast": "Hard", "Image Saturation": "Hard", "Image Sharpness": "Hard", "Image PrintIM": "[80, 114, 105, 110, 116, 73, 77, 0, 48, 51, 48, 48, 0, 0, 6, 0, 1, 0, 22, 0, ... ]", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail Orientation": "Rotated 90 CW", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "30616", "Thumbnail JPEGInterchangeFormatLength": "4051", "Thumbnail YCbCrPositioning": "Co-sited", "EXIF ExposureTime": "1/80", "EXIF FNumber": "9", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "100", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2009:04:02 11:58:43", "EXIF DateTimeDigitized": "2009:04:02 11:58:43", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "8", "EXIF BrightnessValue": "31/4", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "361/100", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Daylight", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "18", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "3056", "EXIF ExifImageLength": "4592", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed"}