63 National Parks.
How many have you visited?

Yellowstone sits on one of the world's largest active volcanic systems. Its last major eruption, 640,000 years ago, left a caldera 30 by 45 miles wide.

Why go: Watch Old Faithful erupt and see the bright colors of Grand Prismatic Spring. Look for wolves, bison, and grizzlies, and walk among geysers, hot springs, and mud pots.

The Colorado River carved the canyon over 5-6 million years, exposing rocks 1.8 billion years old. At its deepest point, it drops 6,093 feet.

Why go: Watch the colors change along the rim through the day. Hike to the bottom on the Bright Angel Trail, raft the Colorado River, or watch condors ride the thermals.

Kolob Arch, at 287 feet, is one of the world's largest natural arches. Zion's cliffs rise 2,000 feet above the canyon floor.

Why go: Hike Angels Landing, with chains to hold and 1,500-foot drop-offs. Wade through the Narrows between tall canyon walls and watch sunset light up the sandstone cliffs.

El Capitan is the largest exposed granite monolith in the world, rising 3,000 feet from the valley floor. The park's giant sequoias include trees over 2,000 years old.

Why go: See Yosemite Falls at full flow during the spring snowmelt. Hike to the top of Half Dome on the cables, watch climbers on El Capitan, and walk among giant sequoias.

Acadia was the first national park east of the Mississippi River. From October through March, Cadillac Mountain is the first place in the US to see the sunrise.

Why go: Watch sunrise over the Atlantic from Cadillac Mountain and walk the 45 miles of historic carriage roads. Climb the iron rungs of the Precipice Trail and eat lobster in Bar Harbor.

Denali, at 20,310 feet, is the tallest peak in North America. The park covers 6 million acres but has only one road.

Why go: Look for the "Big Five": moose, caribou, wolves, Dall sheep, and grizzly bears. Take a flightseeing tour around the peak and ride the park bus into the backcountry.

This is the most visited national park in the US, with over 12 million visitors a year. The "smoke" is a natural fog given off by the forest.

Why go: Watch thousands of synchronous fireflies blink in unison in early June. Drive the Blue Ridge Parkway, visit historic pioneer settlements, and hike through old forests.

Arches has over 2,000 natural stone arches, the highest concentration in the world. Delicate Arch appears on Utah license plates.

Why go: Hike to Delicate Arch at sunset, when the sandstone glows orange against the La Sal Mountains. Explore the Windows section and see Landscape Arch's 306-foot span.