Before you head up Haleakala, call for the latest park weather conditions (PHONE: 808/877-5111). Extreme gusty winds, heavy rain, and even snow in winter are not uncommon. Because of the high altitude, the mountaintop temperature is often as much as 30 degrees cooler than that at sea level. Be sure to bring a jacket.

You can learn something of the volcano's origins and eruption history at the Park Headquarters/Visitor Center, at a 7,000-foot elevation on Haleakala Highway. Hikers and campers should check-in here before heading up the mountain. Maps, posters, and other memorabilia are available at the gift shop.

Leleiwi Overlook, at about an 8,800-foot elevation on Haleakala, is one of several lookout areas in the park. A short walk to the end of the parking lot reveals your first awe-inspiring view of the crater. The small hills in the basin are volcanic cinder cones (called pu'u  in Hawaiian), each with a small crater at its top, and each the site of a former eruption. If you're here in the late afternoon, it's possible you'll experience a phenomenon called the Brocken Specter. Named after a similar occurrence in East Germany's Harz Mountains, the "specter" allows you to see yourself reflected on the clouds and encircled by a rainbow. Don't wait all day for this, because it's not a daily occurrence.

The famous silversword plant grows amid the desert like surroundings at Kalahaku Overlook, at the 9,000-foot level on Haleakala. This odd, endangered beauty grows only here at this summit of this mountain, and at the same elevation on the Big Island's two peaks. It begins life as a silver, spiny-leaf rosette and is the sole home of a variety of native insects (it's the only shelter around). The silversword reaches maturity between 7 and 17 years, when it sends forth a 3- to 8-foot-tall stalk with several hundred tiny sunflowers. It blooms once, then dies.

The Haleakala Visitor Center, at an elevation of 9,740 feet, has exhibits inside, and a trail from here leads to White Hill -- a short, easy walk that will give you an even better view of the valley. Hosmer Grove, just off the highway before you get to the visitor center, has campsites and interpretive trails. Park rangers maintain a changing schedule of talks and hikes both here and at the top of the mountain, including an hour-long loop trail into the Waikamoi Cloud Forest that will give you insight into Hawai'i's fragile ecology. Call the park for current schedules.

Just before the summit, the Crater Observatory offers warmth and shelter, informative displays, and an eye-popping view of the cinder-cone-studded, 7-mi-by-3-mi crater. The highest point on Maui is the Pu'u 'Ula'ula Overlook, at the 10,023-foot summit. Here you'll find a glass-enclosed lookout with a 360-degree view. The building is open 24 hours a day, and this is where visitors gather for the best sunrise view. Dawn begins between 5:45 and 7, depending on the time of year. On a clear day you can see the islands of Moloka'i, Lana'i, Kaho'olawe, and Hawai'i (the Big Island). On a really  clear day you can even spot O'ahu glimmering in the distance.

The air is very thin at 10,000 feet. Don't be surprised if you feel a little breathless while walking around the summit. Take it easy and drink lots of water. Anyone who has been scuba diving within the last 24 hours should not make the trip up Haleakala.

On a small hill nearby, you'll see Science City, an off-limits research and communications center straight out of an espionage thriller. The University of Hawai'i maintains an observatory here, and the Department of Defense tracks satellites. www.nps.gov/hale. COST: $10 per car, good for one week. OPEN: Park Headquarters/Visitor Center daily 8-4, Haleakala Visitor Center daily sunrise-3.