The Camuy Caves, near Arecibo on the north coast, is the third-largest cave system in the world. There are well-paved access roads, a reception area, and electric trains to the entrance of the caves. The Arecibo Observatory is the site of the largest radar/radio telescope in the world. Located in the unusual karst country of Puerto Rico, the 20-acre dish is best seen from a small airplane flight between San Juan and Mayagüez.
The Caguana Indian Ceremonial Park, south of the Arecibo Observatory, was built by Taino Indians as a site for recreation and worship 800 years ago. There is another Ceremonial Park in Ponce.