Reinforcing the division of South America are the Gran Chaco, a semi-arid, sparsely populated lowland region roughly at the center of the continent; the wetlands of the Pantanal; and the Atacama Desert. They are as difficult to traverse as they are inhospitable to human settlement.
South America’s largest and most important cities are therefore found primarily on the coasts. Their situation is particularly pronounced on the Pacific Coast, where there is little room between the ocean and the Andes. Cities along the Atlantic Coast have a little more breathing room – people were able to settle in the Rio de la Plata Basin, which boasts fertile soil, useful river systems and hospitable climate and terrain – but they are nonetheless densest near the ocean.