Mauna Ke'a is the antithesis of the typical island experience. Freezing temperatures and arctic conditions are common at the summit, and snowstorms can occur year-round. It's also home to Lake Waiau, one of the highest lakes in the world. The summit -- at 13,796 feet -- is reputedly the clearest place in the world for viewing the night sky; it's also an outstanding place to see the sun rise and set. To get there, you'll need a four-wheel-drive vehicle. Make sure to stop at the Onizuka Center for International Astronomy Visitor Information Station (PHONE: 808/961-2180. www.ifa.hawaii.edu/info/vis. OPEN: Daily 9  AM-10  PM), at a 9,300-foot elevation. On weekends the Onizuka Center offers escorted summit tours, heading up the mountain in a caravan. Reservations are not required for the free tours, which depart at 1  PM. The center is the best amateur observation site on the planet, with three telescopes and a knowledgeable staff. It hosts nightly stargazing sessions from 6 to 10. To get here from Hilo, which is about 34 mi away, take Highway 200 (Saddle Road), and turn right at mile marker 28 onto the John A. Burns Way, which is the access road to the summit.

If you haven't rented a four-wheel-drive vehicle, don't want to deal with driving to the summit, or don't want to leave until you see the stars (the Onizuka Center tour gets everyone off the summit no later than a half hour after sunset), consider booking a tour. All of the ones listed here provide parkas, as well as telescopes, and meals; excursion fees range from about $90 to $165. Arnott's Lodge & Hiking Adventures (PHONE: 808/969-7097. www.arnottslodge.com) leaves from Hilo and is a bit cheaper than the others. Hawai'i Forest & Trail (PHONE: 808/331-8505 or 800/464-1993. www.hawaii-forest.com) stops for dinner along the way at a historic ranch. Mauna Kea Summit Adventures (PHONE: 808/322-2366. www.maunakea.com) specializes in tours to the mountain. It was the first company to do so, so it has a bit more cred than the rest of the pack.

Whether you're hiking or driving to the summit, take the change in altitude seriously -- don't overexert yourself, especially at the top. Note that scuba divers must wait at least 24 hours before attempting a trip to the summit to avoid getting the bends.