The _ is continuous movement of water between earth's surface and the air

The water cycle or hydrologic cycle is the continuous movement of water between the earth's surface and the air. Water molecules move from the atmosphere (or air) to the earth's surface and back again. Water moves from the atmosphere to the earth's surface via rain, snow, sleet or hail. It moves back to the atmosphere via evaporation. The sun's energy is the catalyst for the movement of water between the atmosphere and the earth's surface.

Which one is the underlined word?