adjective
An adjective is a word that describes or modifies a noun or pronoun. In this case, 'little' is an adjective that describes the noun 'known'.
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There are many little known facts about the history of this ancient civilization.
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Did you know there are some little known secrets about our favorite celebrity?
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I'll tell you a little known trick to ace that exam.
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The author's novel explores the little known depths of human emotions.