Little-Known Facts About America's National Parks (PHOTOS)

National Park Week is underway, and free entrance to nearly 400 parks continues through this Sunday.

In celebration of the festivities, the National Park Foundation, the non-profit behind the fee-free week, shared with HuffPost Travel some surprising statistics about what Ken Burns calls "America's Best Idea."

• 84 million: The total acreage of the parks, an area roughly the size of Germany.
• 281 million: The average number of people that visit a park every year.
• 18,000: Miles of trails traverse the parks.
• 45,000: People who volunteer at America's national parks.
• 13.2 million: The acreage of the largest park, Wrangell-St. Elias in Alaska. (It covers an area larger than Costa Rica.)
• .02: The acreage of the smallest, Thaddeus Kosciuszko National Memorial in Pennsylvania. (He was a Polish soldier who joined the Continental Army during American Revolution.)
• 0: The number of national parks in Delaware, the only state without one.

What park gets the most visitors? The Blue Ridge Parkway, the 469-mile drive that threads the Appalachian Mountains of North Carolina and Virginia.