Yemen is one of the world's most water-scarce countries, and Sanaa could be the first national capital in the world to completely exhaust its water supply. The city is located on the Tawilah aquifer, which was first identified in 1972. The aquifer has a natural recharge rate of 42 Mm3/a, much of which comes from the periodic outflow of water from the surrounding wadis onto the Sanaa plain. There is not much refill from rainfall. In 1995, water extraction from the aquifer exceeded the natural recharge rate by around 300%. More recent estimates are higher, suggesting 400-500%. This has caused groundwater levels to drop by 6 to 8 metres annually, to the point that many wells have to be drilled as far down as 2,600 to 3,900 feet. It is estimated that, with a slightly lower rate of depletion, the aquifer will be completely exhausted by around 2030.