The boundary between Guatemala and El Salvador extends for 209 kilometers (130 miles) from the tripoint with Honduras at the summit of Cerro Montecristo to the mouth of the Rio Paz in the Pacific Ocean. The boundary follows watersheds, rivers, and other geographic or man-made features in a southwesterly direction. Delimited in detail in 1938 and then fully demarcated in 1940, the modern frontier has remained unchanged in the intervening decades. In 1987, El Salvador, Guatemala, and Honduras established a biosphere reserve adjacent to the tripoint on Cerro Montecristo. In the early 2000s, El Salvador, Guatemala, Honduras, and Nicaragua agreed to open up their borders allowing for the free movement of people and the creation of a universal visa for their citizens.