Water is one of the key ingredients to life on Earth. About 75 percent of our planet is covered by water or ice. The water cycle is the endless process that connects all of that water. It joins the Earth’s oceans, land, and atmosphere.

The Earth’s water cycle began about 3.8 billion years ago when rain fell on a cooling Earth, forming the oceans. The rain came from water vapor that escaped the magma in the Earth’s molten core into the atmosphere. Energy from the sun helped power the water cycle and Earth’s gravity kept water in the atmosphere from leaving the planet.

The oceans hold about 97 percent of the water on Earth. About 1.7 percent of Earth’s water is stored in polar ice caps and glaciers. Rivers, lakes, and soil hold approximately 1.7 percent. A tiny fraction—just 0.001 percent—exists in the Earth’s atmosphere as water vapor.

When molecules of water vapor return to liquid or solid form, they create cloud droplets that can fall back to Earth as rain or snow—a process called condensation. Most precipitation lands in the oceans. Precipitation that falls onto land flows into rivers, streams, and lakes. Some of it seeps into the soil where it is held underground as groundwater.

When warmed by the sun, water on the surface of oceans and freshwater bodies evaporates, forming a vapor. Water vapor rises into the atmosphere, where it condenses, forming clouds. It then falls back to the ground as precipitation. Moisture can also enter the atmosphere directly from ice or snow. In a process called sublimation, solid water, such as ice or snow, can transform directly into water vapor without first becoming a liquid.