The Río de la Plata (literally "Silver River") and the modern country of Argentina (from the Latin argentum, "silver") both take their names from the myth. The legend of the Sierra de la Plata may have been based on the Cerro Rico de Potosí in Bolivia, which was discovered by a Spanish expedition traveling from Peru in 1545.
The legend of the White King and the Sierra de la Plata began with the expeditions of Juan Díaz de Solís along the coast of South America. On his first voyage in 1512, Solís followed the coast of Brazil until he came across an enormous estuary, the Río de la Plata, which Amerigo Vespucci had named the River Jordan on his 1501-02 expedition and the local inhabitants called Paranaguazu ("river like the sea" or "great water"). Solís decided to call it the Mar Dulce ("Freshwater Sea") due to its great size. After exploring the area and guessing it could be a strait connecting the Atlantic to the Pacific, Solís returned to Spain to stake his claim as conqueror and governor of the region. In 1516, he returned with the title of Captain General, but when Solís and his party landed on the eastern bank of the Río de la Plata, they were attacked and killed by Guaranís. Seeing this, the crew remaining on the ships decided to weigh anchor and return to Spain.
On their way back to Europe, one of the Solís expedition's vessels shipwrecked off the coast of Santa Catarina Island in what is now Brazil, leaving eighteen men stranded. One of them, the Portuguese explorer Aleixo Garcia, became friendly with the local Tupí-Guaranís, and through them learned of a great mountain of shining metals far into the mainland.
Garcia left Santa Catarina along with other castaways and a large indigenous party to search for the Sierra de la Plata, crossing most of South America before reaching the Andean altiplano. This was supposedly the home of the White King, whose throne was entirely decorated with silver. After taking a few valuable pieces, the explorers headed back to the Brazilian coast, but along the way, Aleixo Garcia and the other Europeans were killed in a Payaguá ambush. The few Tupí-Guaranís who managed to escape told their story, showing off the silver pieces they had gotten from the realms of the White King.
In 1526, the Venetian explorer Sebastian Cabot left Spain with the goal of reaching the Molucca Islands in Indonesia by way of the Straits of Magellan. During a stopover in Pernambuco in northern Brazil, he first heard the story about a land rich in precious metals far inland, which could be reached via an enormous estuary further south. The estuary ended up being called the Río de la Plata for its role as the supposed natural gateway to the treasure. The legend captivated Cabot, so he abandoned his mission and decided to find the Sierra de la Plata, assuming that the royal authorities would be indulgent if he found enough silver.