In limestone caves, after the natural process of erosion and excavation, a simple but slow natural process is responsible for the decoration of the bare, dull walls. Falling rain picks up atmospheric carbon dioxide. On passing through the soil, more carbon dioxide, from plant roots and decaying vegetable matter, becomes dissolved in the water, along with complex organic acids called humic acids. This ground water easily disolves limestone. The solution slowly descends into the cave and deposits the dissolved calcium carbonate, sometimes including impurities of iron and other minerals. On reaching the cave, the water droplets fall to the floor, leaving behind a tiny deposit of calcite crystal. This is known as precipitation.