Esperanza Stone

The Esperanza Stone was a large (8-feet long) inscribed stone found in the valley of the Yaqui, Mexico. It was discovered and excavated in 1909 by Major F. R. Burnham and Charles Frederick Holder. Burnham believed that the symbols were Mayan. Others class them as petroglyphs.
More information on the Wikipedia page . 
Mentions here, here, here and here. There is various other coverage.
Category:Prehistoric inscriptions
Category:20th century