In Illinois, the one-hundred-foot Cahokia Mound spreads impressively across sixteen acres, and as many as ten thousand more mounds dot the Ohio River Valley alone. The Mound Builders traces the speculation surrounding these monuments and the scientific excavations which uncovered the history and culture of the ancient Americans who built them. The mounds were constructed for religious and secular purposes some time between 1000 B.C. and 1000 A.D., and they have prompted curiosity and speculation from very early times. European settlers found them evidence of some ancient and glorious people. Even as eminent an American as Thomas Jefferson joined the controversy, though his conclusions—that the mounds were actually cemeteries of ancient Indians—remained unpopular for nearly a century. Only in the late 19th century, as Smithsonian Institution investigators developed careful methodologies and reliable records, did the period of scientific investigation of the mounds and their builders begin. Silverberg follows these excavations and then recounts the story they revealed of the origins, development, and demise of the mound builder culture.