For millions of years the Pacific plate has been colliding with the Nazca plate, pushing up the Andean High plateau. At one such juncture, what is now in modern day Bolivia, it created an ancient inland salt sea that sat in a geological bathtub between two parallel high Andean ranges. In more recent transformations of the past thousands of years, it has been reduced to form the largest salt flat in the world at more than 4,000 square miles. Neighboring Lake Titicaca, which spans the Bolivian and Peruvian border, is also part of this ancient Inland sea, yet filled with water, making it the highest navigable lake in the world.