10. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: obtaining a plurality of seed facts, wherein each seed fact identifies a subject entity, an attribute possessed by the subject entity, and an object, and wherein the object is an attribute value of the attribute possessed by the subject entity; generating a plurality of patterns from the seed facts, wherein each of the plurality of patterns is a dependency pattern generated from a dependency parse, wherein a dependency parse of a text portion corresponds to a directed graph of vertices and edges, wherein each vertex represents a token in the text portion and each edge represents a syntactic relationship between tokens represented by vertices connected by the edge, wherein each vertex is associated with the token represented by the vertex and a part of speech tag, and wherein a dependency pattern corresponds to a sub-graph of a dependency parse with one or more of the vertices in the sub-graph having a token associated with the vertex replaced by a variable; applying the patterns to documents in a collection of documents to extract a plurality of candidate additional facts from the collection of documents, wherein applying the patterns to documents in the collection of documents comprises: applying the dependency patterns to documents from the collection of documents to identify matching sentences; generating an extraction from each matching sentence; and aggregating the extractions to generate the candidate additional facts; and selecting one or more additional facts from the plurality of candidate additional facts.
10. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: obtaining a plurality of seed facts, wherein each seed fact identifies a subject entity, an attribute possessed by the subject entity, and an object, and wherein the object is an attribute value of the attribute possessed by the subject entity; generating a plurality of patterns from the seed facts, wherein each of the plurality of patterns is a dependency pattern generated from a dependency parse, wherein a dependency parse of a text portion corresponds to a directed graph of vertices and edges, wherein each vertex represents a token in the text portion and each edge represents a syntactic relationship between tokens represented by vertices connected by the edge, wherein each vertex is associated with the token represented by the vertex and a part of speech tag, and wherein a dependency pattern corresponds to a sub-graph of a dependency parse with one or more of the vertices in the sub-graph having a token associated with the vertex replaced by a variable; applying the patterns to documents in a collection of documents to extract a plurality of candidate additional facts from the collection of documents, wherein applying the patterns to documents in the collection of documents comprises: applying the dependency patterns to documents from the collection of documents to identify matching sentences; generating an extraction from each matching sentence; and aggregating the extractions to generate the candidate additional facts; and selecting one or more additional facts from the plurality of candidate additional facts. 13. The system of claim 10 , wherein generating the plurality of patterns from the seed facts comprises, for each of the seed facts: identifying sentences in the collection of documents that match the seed fact; identifying a respective minimal sub-graph of a dependency parse of each of the matching sentences, wherein the minimal sub-graph of a dependency parse is a smallest portion of the dependency parse that includes vertices representing head tokens of a subject entity, attribute, and object identified by the seed fact; and generating a respective dependency pattern from each minimal sub-graph by replacing the tokens associated with one or more of the vertices representing the head tokens of the subject entity, the attribute, or the object identified by the seed fact with a variable.