An aquifer is an underground layer of porous material which contains water (groundwater), which can be extracted with a well. It can be permeable rock, gravel, sand, silt, or clay.
The water may be recharged by local rainfall and/or travel long distances underground - up to thousands of kilometers. There are aquifers in lowland south Asia, such as Bengal and Bangladesh, which originate from the Himalayas, and the long periods underground in certain soil types lead to the problem of arsenic in groundwater.