Build the National Park to eliminate all unhealthiness from population in your city. It also provides a free specialist for each Forest Preserve in its city radius and increases the chances that a Great Scientist might appear in your city.
While areas of great natural beauty had often been protected or revered in the past, the modern conception of a National Park did not appear until 1872. Yellowstone National Park, located in the western United States, departed from previous nature preserves - which tended to be set aside for nobility - by opening up the land for the enjoyment of the public. After the creation of Yellowstone, National Parks began popping up throughout the world.