Continuing advances in the archaeology of the Amazon have changed long-standing misconceptions about the rainforest as a homogeneous, nearly pristine environment occupied by small, scattered groups. Massive archaeological sites, deep deposits of anthropogenic soils, and earthworks found over thousands of kilometers now testify to the scale and intensity of past human impact in some parts of the Amazon. However, debate persists about the extent of such transformations, as distinct environments within the Amazon Basin (floodplains, savannas, seasonal forests) reveal different scales and intensities of pre-Columbian landscape modification. In that context, the discovery of hundreds of geometric earthen enclosures in the southern rim of the Amazon is proving that some areas that were previously considered virtually untouched forest may have been densely settled in the past. Although regional variations exist, most southern Amazonian enclosures appear to be defensive earthworks built at the turn of the second millennium ce, a period recognized by archaeologists as one of escalating population densities, migrations, and warfare across the Amazon Basin.