Rain and other forms of precipitation fall on our planet as slightly acidic fresh water. As some of this water washes over the landscape, it physically erodes rocks and chemically breaks them down. It then carries salts from the rocks into rivers and other streams and deposits the salts in the ocean. Most inland bodies of water remain fresh because they are continually replenished by precipitation and have at least one outlet to keep things moving through them, so the minerals in them do not reach high concentrations. However, the ocean collects all the salts from all the streams that flow into it, and, when water evaporates from the ocean as part of the water cycle, those salty chemicals get left behind. This cycle of mineral deposition and accumulation, having occurred for millennia, has concentrated the salts in the ocean so much that it is no longer fresh but saline. Hydrothermal vents on the ocean floor and undersea volcanic activity also contribute salts to seawater.