There exists a lot of different reading material on the subject, and many of them are somewhat similar. One of the most well known, as far as I know, is the work by Zecharia Sitchin, where he apparently translated some really old stone tablets speaking of deities of the ancient Sumeria. He translated the deities as the "Annunaki". The translations is worth reading imho, "The lost book of Enki" ISBN 9781879181830.
No, I do not say that this is the truth, but it's a fairly fascinating story nonetheless.
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