Any Americans at a loose end tomorrow might want to think about heading out to a National Park and helping the United States National Park Service (NPS) celebrate its 98th birthday. Entry and tours will be free and if you make the right choice of park you might get to join a party. 

A third of Americas 401 National Parks charge an entrance fee, and there is also usually a cost for parking. However, under the Free Entrance Day program participating parks waive their fees. Although there are six free entrance days a year, none of the others are in summer, something to consider if you're thinking of visiting, for example, Denali National Park, Alaska. 

The NPS was established on August 25, 1916 to "to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations," However, national parks predate the service by 44 years, with Yellowstone established in 1872 long before it became clear a service would be needed to preserve such places.

From the beginning science was intimately bound up with parks. Yellowstone was chosen for the chance to study volcanic processes and mountain building. Subsequently parks became a chance to preserve and investigate both individual species and the ecological webs in which they live.

However, the political will to maintain parks against encroachment from agriculture and mining depends in large part on tourism, both as an important part of the economy around many of the parks and to provide a voting block of people keen to protect. With 275 million park visits each year, those who love parks are a force to be reckoned with, but the NPS are keen to increase that further.

Non-Americans might want to start making plans to visit in 2016, when the centenary celebrations will be in full swing.

^{Guy Francis. U-shaped valley, Yosemite. Another of the parks that normally charges but is free on Monday.}

H/T LivesScience