Illustration by Elizabeth Brockway/The Daily Beast/GettyLast September, New York City’s Swann Galleries were advertising the sale of an invaluable piece of Spanish and Mexican history: a 500-year-old letter involving Hernán Cortés, the Spanish military leader and colonizer. The letter was expected to sell for somewhere between $20,000 and $30,000 until a group of academics intervened. Reuters reports that the letter was one of a cluster of Cortés documents that had been stolen out of the National Archive of Mexico (AGN) and put up for sale. What’s even more shocking is that this is not the first time that important and valuable pieces of history have been stolen from a national archive, prominent library, or museum and ended up on the block at a prominent auction house.The thefts would have gone unnoticed had it not been for the investigations of amateur sleuths and professional academics María Isabel Grañén Porrúa, a scholar of Spanish