Earth's troposphere, the lowest layer of the atmosphere, contains particles of dust, ash, ice, and salt. Particulate matter in the atmosphere can come from the soil, volcanic eruptions, and the ocean. These airborne particles are sometimes carried into the atmosphere by the wind. The wind transports salt particles from the ocean and ocean spray into the atmosphere. Fungi, bacteria, and other microscopic organisms can also be found in the atmosphere, attached to dust and other particulates. Water droplets and ice in the atmosphere collect around these pieces of dust, dirt, ash, etc., which are known as condensation nuclei, to form clouds.