San Juan is known as "La Ciudad Amurallada" (the walled city). San Juan is one of the largest natural harbors in the Caribbean and is the second oldest city in America. The metropolitan area known as San Juan has 3 distinct areas: Old San Juan, the Beach & Resort area, and other outlying communities. Old San Juan is a 7-square residential and commercial block. The streets are paved with cobbles of adoquine, a blue stone cast from furnace slag. The city has more than 400 restored Spanish buildings from the 16th and 17th century.