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The Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geological structure in Ontario, Canada. It is the third-largest known impact structure on Earth, as well as one of the oldest. The structure, the eroded remnant of an impact crater, was formed by the impact of an asteroid 1.849 billion years ago in the Paleoproterozoic era. The basin is located on the Canadian Shield in the city of Greater Sudbury, Ontario. The former municipalities of Rayside-Balfour, Valley East and Capreol lie within the Sudbury Basin, which is referred to locally as "The Valley". The urban core of the former city of Sudbury lies on the southern outskirts of the basin. An Ontario Historical Plaque was erected by the province to commemorate the discovery of the Sudbury Basin.