A khettara is a drain gallery designed to transport groundwater from the mountains to agricultural land. It usually begins near a well in the foothills of the mountains where underground water is available but cultivable soil is not. The water moves by gravity from this place through a tunnel to an oasis. The watermark at the start may be at 20 meters (65 feet) and the length of the khettaras can reach 20 kilometers (12 miles). To maintain the gallery, each 10 to 15 meters (32 to 50 feet) a shaft is dug. Khettaras are centuries old and need constant maintenance. The construction of a khettara is a continuing process. In former years they used to be shorter but as soon as the water level lowers they need to be extended otherwise they run dry.