Mud is a scientific, as well as familiar, term for wet, slimy, or sticky debris produced by rain on the Earth's surface, ejections from springs or volcanoes, and the settling of turbid waters. Horn and her students study mud that accumulates at the bottoms of lakes and in bogs and swamps in the southeastern United States and Latin America. Ancient mud provides clues about past climate, vegetation, wildfires, other natural disturbances, and human land use and impacts. Mud samples are recovered in ways that preserve the stratigraphy of deposits. Radiocarbon dating and other techniques are used to determine sample ages. From microfossils such as pollen, particle sizes and shapes, and geochemical signatures, we develop records of environmental history that complement and extend what we can lean from archaeology and from other Earth archives—such as tree rings.