In the Commonwealth of Puerto Rico, George Bauer visits San Juan, where he explores Old San Juan, with its narrow streets, charming old buildings and Spanish fortifications. Other highlights include Arecibo Observatory, site of the world's largest radio telescope; Rio Camuy Cave Park, containing the world's third-largest underground river and caves that are 45 million years old; and the rugged Sierra de Luquillo, site of the Caribbean National Forest, which gives new meaning to the term "rain forest."