If you associate Puerto Rico's capital with the colonial streets of Old San Juan, then you know only part of the picture. San Juan is a major metropolis, radiating out from the bay on the Atlantic Ocean that was discovered by Juan Ponce de León. More than a third of the island's nearly 4 million citizens proudly call themselves sanjuaneros. The city may be rooted in the past, but it has its eye on the future. Locals go about their business surrounded by colonial architecture and towering modern structures.By 1508 the explorer Juan Ponce de León had established a colony in an area now known as Caparra, southeast of present-day San Juan. He later moved the settlement north to a more hospitable peninsular location. In 1521, after he became the first colonial governor, Ponce de León switched the name of the island—which was then called San Juan Bautista in honor of St. John the Baptist—with that of the settlement of Puerto Rico ("rich port").Defended by the imposing Castillo San Felipe del Morro (El Morro) and Castillo San Cristóbal, Puerto Rico's administrative and population center remained firmly in Spain's hands until 1898, when it came under U.S. control after the Spanish-American War. Centuries of Spanish rule left an indelible imprint on the city, particularly in the walled area now known as Old San Juan. The area is filled with cobblestone streets and brightly painted, colonial-era structures, and its fortifications have been designated a UNESCO World Heritage Site.Old San Juan is a monument to the past, but most of the rest of the city is planted firmly in the 21st century and draws migrants island-wide and from farther afield to jobs in its businesses and industries. The city captivates residents and visitors alike with its vibrant lifestyle as well as its balmy beaches, pulsing nightclubs, globe-spanning restaurants, and world-class museums. Once you set foot in this city, you may never want to leave.