This article originally appeared in Conde Nast Traveler.

White sand beaches, swamps, stalagmite-filled caves, glaciers, hot springs you can swim in (and ones you definitely can't), rainforests, scorching deserts, volcanoes (dormant and active): The national parks scattered throughout the United States and its territories truly have it all. And while the 63 recognized parks only make up a fraction of the 423 sites that comprise the National Park Service, they represent some of the finest natural environments America has to offer. (And for six days a year, visiting these parks is free, too.)

Here, our national parks, from A to Z.