The second zoo to open in the United States, the Cincinnati Zoo was named a national historic landmark in 1974. Students will feel as though they are strolling through the 75-acre (30-ha) grounds, as they read about the more than 700 species of animals and 3,000 species of plants. Children will read about the creepy-crawly critters in the World of Insects, and about the gentle manatees, Stoneman and Douglas, at Manatee Springs.