The Olmec colossal heads are at least seventeen monumental stone representations of human heads sculpted from large basalt boulders. The heads date from at least before 900 BC and are a distinctive feature of the Olmec civilization of ancient Mesoamerica. All portray mature men with fleshy cheeks, flat noses, and slightly crossed eyes; their physical characteristics correspond to a type that is still common among the inhabitants of Tabasco and Veracruz. The backs of the monuments often are flat. The boulders were brought from the Sierra de los Tuxtlas mountains of Veracruz. Given that the extremely large slabs of stone used in their production were transported over large distances, requiring a great deal of human effort and resources, it is thought that the monuments represent portraits of powerful individual Olmec rulers. Each of the known examples has a distinctive headdress. The heads were variously arranged in lines or groups at major Olmec centres, but the method and logistics used to transport the stone to these sites remain unclear.