The observatory — the closest you can get to the crater by car — is 5 1/2 miles from the crater and 4,300 feet above sea level, giving visitors a front-row view of the steaming cavity. The 16,000-square-foot concrete-and glass structure is crammed with enlightening exhibits, including a wide-screen theater presentation of the 1980 eruption.

Directions, hours: From Interstate 5, take Washington Highway 504 east for 52 miles until it ends. Open 10 a.m.6 p.m. May through October. Fee: $8 adults; free for those 16 and younger.

From the observatory, follow this gently sloping, paved, half-mile path to view the effects of the eruption, including shattered trees and vistas of the lava dome, its crater, a pumice plain and landslide deposits.

Directions: From Interstate 5, take Highway 504, 52 miles east of Castle Rock to the Johnston Ridge Observatory.

More strenuous than the Eruption Trail, the Hummocks Trail makes a 2.3-mile loop through the valley between the observatory and the volcano. The path borders 100- to 200-foot mounds of earth the volcano shoved from the peak in the last eruption.

Directions: From I-5 take the 504 east and continue until you see milepost 45. Look for a parking lot on your right.

Here's your chance to explore an ancient lava tube that seems to burrow into the center of the Earth. This 2.5-mile cave was formed nearly 2,000 years ago when an outside layer of lava cooled while hotter lava inside continued to flow. Wear warm clothes and sturdy shoes and take a flashlight.

Directions: From I-5, take Washington Highway 503 east for about 36 miles until it becomes U.S. Forest Service Road 90, and then turn left on U.S. Fire Road 83 for about two miles and look for signs.

The 1980 eruption wiped out four hugely popular campsites around Spirit Lake, north of the crater. Now one of the closest waterfront campgrounds is Cougar Park Campgrounds, on the shores of Yale Reservoir. Campers here enjoy dozens of campsites, showers, picnic tables, a boat ramp and a protected swimming area.

Directions: From I-5, take the 503 east about 33 miles and look for signs for Cougar Camp on your right after passing the town of Cougar. Fees: $17 per night.

The 1980 eruption obliterated hundreds of acres of timberland, much of which was replanted by the Weyerhaeuser timber company a few years later. Because the noble and Douglas fir seedlings took root at the same time, the new trees have a beautiful uniformity — all about the same height and width. Amid the trees, Weyerhaeuser built a learning center where visitors can enter a multimedia "eruption chamber" that simulates the sound and feel of a volcano.

Directions: From I-5, take the 504 east to mile marker 33 and look for signs. Fee: none.