Rio de la Plata

Composing part of the border between Argentina and Uruguay, this 180-mile-long estuary is formed by the confluence of the Uruguay River and the Paraná River. Though used for centuries by native tribes, the river wasn’t explored by Europeans until the 16th-century Spanish navigator Juan Díaz de Solís went in search of a passage between the Atlantic and the Pacific Oceans. He originally named this enclosed coastal body, about one mile across at its widest point, the Mar Dulce, or freshwater sea; after multiple explorations by various other Spanish navigators, the waterway came to be known as the Rio de la Plata, or River of Silver, for the promise of riches thought to lie upstream.