text:  The <unk> 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 <unk> civilization of ancient <unk> . All portray mature men with <unk> <unk> , flat <unk> , and slightly crossed eyes ; their physical characteristics correspond to a type that is still common among the inhabitants of <unk> and Veracruz . The backs of the monuments often are flat . The boulders were brought from the Sierra de los <unk> 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 <unk> rulers . Each of the known examples has a distinctive headdress . The heads were variously arranged in lines or groups at major <unk> centres , but the method and logistics used to transport the stone to these sites remain unclear . 
  