How do desert oases form?

An oasis is a place in a desert where water is available at or near the surface, allowing plants and people to survive there. Many oases are fed by underground aquifers that emerge naturally through springs, while others are created where faults, rivers, or impermeable rock layers force groundwater upward. Some oases are linked to fossil groundwater that fell as rain thousands of years ago during wetter climate periods. Because they concentrate water and vegetation, oases have long supported travel routes and settlements across deserts such as the Sahara and Arabian Peninsula. Their size can range from a small spring to a whole valley of cultivated land.