more (at source): The plaza may contain other artifacts dating from 600 A.D. to 1500 A.D., including piles of refuse from daily life, Rivera said.

"I have visited many sites and have never seen a plaza of that magnitude and of those dimensions and with such elaborate petroglyphs," said Miguel Rodriguez, member of the government's archaeological council and director of a graduate school in Puerto Rico that specializes in history and humanities. He is not involved in the excavation project.

Archeologists have known since 1985 that the area contained indigenous artifacts. But their extent and significance only became clear this month when the U.S. Army Corps of Engineers began work on removing them so the land could be used for a dam project.