Oasis is a fertile, vegetated area in a desert where underground water comes close enough to the surface for wells and springs to exist. Water that occurs at an oasis originally fell as rain or snow in distant mountains or hills. After seeping into the ground, the water slowly filters through the underground rocks toward low places in the desert. It then reappears at the surface to form springs or is shallow enough to be found in wells. Fertile, vegetated areas along permanent streams in deserts are sometimes also called oases.
Soil in desert regions is generally fertile but lacks the moisture to encourage plant growth. Because oases have water, almost all of them are areas of farming and settlement. Some oases are small and can support only a few people, but others are large enough to support millions of people.
