Guano was widely used by the native populations of pre-Spanish Latin America for centuries as a fertilizer to increase crop yields. However, it was not until the early 1800s that guano was rediscovered by the Europeans to have valuable agricultural benefits as a fertilizer. The best source of guano was discovered on a series of islands off the coast of Peruwhich are barren and rocky with no vegetation due to lack of rain in the area. Peru's primary guano islands are the Chinchas, the Ballestras, the Lobos, and the Macabi and Guanape islands. Peru's guano was considered to be the best for farming. Guano is made up of bird droppings amassed over hundreds of years due to weather and ocean currents. What distinguished Peru's guano from guano found at other sources around the world was due to the unique weather conditions found along Peru's coast. Because of the Humboldt or Peruvian Current, which flows cold water from Antarctica to the equator along Peru's coast, this creates an interesting weather pattern where the cold water and warm air prevents the fall of rain in this part of the world. Due to the lack of rain on the islands along Peru's coast, the accumulated bird droppings are baked in the dry atmosphere which preserves the nitrates in those droppings from evaporating, thus maintaining its effectiveness. Another factor that made guano an effective fertilizer was that its contents originated from fish-eating birds. Remnants of the guano industry can be seen on the Ballestas Islands.