What is now known as Old San Juan occupied the western end of a rocky islet, the Isleta de San Juan, at the mouth of San Juan Bay. During the 20th century, the main population centers surged well beyond the walls of the old city and onto Puerto Rico's main island, and merged with the existing settlements east and south of Old San Juan. With the annexation of Río Piedras in 1951, the municipality of San Juan grew four times its previous size. As a result, the municipality also went from 2 to 18 barrios (barrios), 16 of which fall within the former municipality of Río Piedras. Eight of the 18 barrios are further subdivided into subbarrios, including the two barrios (San Juan Antiguo and Santurce) that belonged to the original municipality of San Juan. The 18 barrios are: