Annual precipitation for Jeju amounts to 3.952 billion tons, of which 24.5% (900.7 million tons) flows into the ocean, 34.9% (1.379 billion tons) evaporates into the air, and the remaining 40.6% (1.604 billion tons) is absorbed into the ground and becomes groundwater. The groundwater is purified as it passes through multiple layers of volcanic rock and turns into clean volcanic water.