Wikipedia
Tanka is a traditional rainwater harvesting technique, common to the Thar desert region of Rajasthan, India. It is meant to provide drinking water for a single or a small group of families and is an important element of water security in these arid regions. A Taanka is a cylindrical underground rainwater storage cistern usually 10’ x 10’ in size, wherein rainwater from rooftops, a courtyard or natural or artificially prepared catchment flows into the paved underground pit, through filtered inlets made on the external wall of the structure, where it is stored and can be used by one family during the dry season. Once fully filled, the water is sufficient for a family of 5-6 members for a period of 5–6 months, and saves it from everyday-water-fetching-drudgery.
The technique was largely abandoned in later 20th century as pipes lines or hand pumps were laid, it was when faced with drought like situations, inadequate supplies of piped water on the account of growing population, which also resulted in depleted or contaminated ground water, this traditional method was revived, along with other traditional rainwater harvesting structures like, Naadi, a village pond and Beri, a small rainwater-collecting wells, especially for supplying drinking water.