Abstract The late Carboniferous (Middle Pennsylvanian, ∼307 Ma ago) Mazon Creek Lagerstätte found in northern Illinois, USA, is unique for its exceptional biotic diversity as well as the human endeavours, both professional and avocational, that brought vast numbers of fossils and new species to science. In 1997, the Mazon Creek Fossil Beds, exposed along the Mazon River near Benson Road, Morris, IL, became a National Historic Landmark. The fossils are preserved in siderite (iron carbonate, FeCO 3) concretions within the lower 3–8 m of the Francis Creek Shale Member of the Carbondale Formation, and often retain outlines of original soft tissues. The Mazon Creek biota includes over 465 animal and 350 plant species representing more than 100 orders, which is attributed to the preservation of organisms from multiple habitats and the large number of specimens collected. That phenomenon was made possible by coal extraction bringing concretions to the surface and highly motivated amateur collectors pursuing them. Some of the formerly mined areas continue to draw collectors and are preserved as part of the Mazonia-Braidwood State Fish and Wildlife Area. The fossils, fossil collectors and collection sites are a significant part of the cultural and scientific geoheritage.