YAGO is a semantic knowledge base of over 10 million entities, and over 120 million facts about those entities, developed by the Max Planck Institut Informatik. Its base derives from Wikipedia, WordNet and GeoNames, and the accuaracy has not only been manually evaluated, but relations are also provided with an annotation stating its confidence value. The YAGO ontology includes spacial and temporal dimensions, and integrates WordNet's thematic domains as well. It can be queried or browsed right from the website, and the full ontology is available for download.
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YAGO was used in this research abstract https://dh2016.adho.org/abstracts/219