An olla (pronounced "oya") is a simple subsurface irrigation device whose design and use dates back many thousands of years. It is an unglazed pot which is buried up to the neck in the soil and filled with water. The wall of the pot is porous, and water seeps through it into into the earth by a process called soil moisture tension. When the soil around the pot is moist, water remains in the pot. As plant transpiration creates a demand for water, more is released through the wall of the olla. This is an extremely efficient process, often resulting in a very significant reduction in water use compared with drip-feed methods.