The discovery of a colossal head at Tres Zapotes in the nineteenth century spurred the first archaeological investigations of Olmec culture by Matthew Stirling in 1938 . Seventeen confirmed examples are known from four sites within the Olmec heartland on the Gulf Coast of Mexico . Most colossal heads were sculpted from spherical boulders but two from San Lorenzo Tenochtitlán were re @-@ carved from massive stone thrones . An additional monument , at Takalik Abaj in Guatemala , is a throne that may have been carved from a colossal head . This is the only known example from outside the Olmec heartland .