Uluṟu–Kata Tjuṯa is normally arid (dry). Most of the water in the park is under the ground. It can stay under the park for thousands of years. In the end, it all drains through the ground to Lake Amadeus in the north. Knowing where to find this underground water has always been important for human survival in the area. It is the only supply of water in the area that people can rely on. Both Muṯitjulu and Yulara get their water from ancient aquifers under the park. The water contains salt, which is removed before it is used. The aquifers being refilled depends on rare major rains, normally happening every ten years or so.