19 incredible photos that show why you should visit a national park this week

The United States is home to some of the most incredible natural sights and wonders in the world, from towering mountains to otherworldly rock formations. The National Park Service celebrates and preserves much of these unique sights, and fosters outdoor exploration for over 300 million guests each year.

April 16th to 24th is National Park Week, meaning that admission to all national parks is free. This year also marks the National Park Service's centennial, as it was created on August 25th, 1916.

From the well-known geysers of Yellowstone to the labyrinthine tunnels of Mammoth Caves, we've rounded up some of the most awe-inspiring and unique features of national parks across the country. Now's as good a time as ever to check one out.