... were, according to Wikipedia, Mesoamerica's first civilisation. They flourished between around 1300 and 500 BC, in what is now the states of Veracruz and Tabasco in south–central Mexico, and are best known today for their artwork – particularly the so–called 'colossal heads'.
The Olmecs' most famous pyramid is the one at La Venta, in the heartland of their territory as described above. It's made of earth and clay. Nobody seems very sure exactly when it was built, but it was abandoned around 400 BC "after 500 years of pre–eminence".
© Macclesfield Quiz League 2020