During the period of Spanish rule an immense revenue accrued from working the rich silver mines. Those that filled the Spanish treasure ships so eagerly sought by buccaneers were the mines of Potosi. These silver lodes, extensively worked through Indian slave labor by Hernando and Gonzalo Pizarro, brothers of Francisco Pizarro, were discovered in 1546. So rich did the lodes prove to be that the city of Potosi sprang up near them and was supported by them, although the site was far from being desirable. Its altitude is about thirteen thousand feet, and it is, therefore, the highest city in the world. It is situated on the bleak side of the Andes, from whose snow-clad peaks cold, piercing winds sweep down over the city. Towering above it is a mountain, honeycombed with shafts, tunnels, and drifts, from which has been taken silver to the value of two billion dollars. At first it was thought that a location so high above sea level would be unhabitable, but the immense wealth of the silver lodes required many workmen for their development, and these laborers had to be housed and fed. At the zenith of its prosperity Potosi possessed one hundred seventy thousand inhabitants, and had the distinction of being the largest city in the New World during the first two centuries of its existence. A mint built in 1562, at the expense of over a million dollars, is long since unused. A splendid granite cathedral ornamented with beautiful statuary still attests to the former grandeur of the city. Some of the richest veins of silver ore in the Potosi mines have been worked out and many mines have been allowed to become filled with water. These conditions, coupled with the low price of silver for many years, have caused the population of the city to dwindle until now there are scarcely more than ten thousand inhabitants and very many of the buildings are in ruins. These mines have produced twenty-seven thousand tons of silver since their discovery, and at the present day many of them are yielding large returns. The Bolivian plateau is one vast mineral bed abounding in rich mines of copper, tin, silver, and gold. In Bolivia alone there are upward of two thousand silver mines; while some of the richest tin mines in the world are found here. Lodes of pure tin several feet in width have been followed down six hundred feet. Tin mines were recently discovered among the mountains thirteen thousand five hundred feet above the level of the sea, near the shores of Lake Titicaca.