Don't miss the opportunity to see how the Hawaiian Islands were made by exploring a million-year-old underground lava tube/cave. Chuck Thorne, of Maui Cave Adventures (tel. 808/248-7308; www.mauicave.com), offers several tours of this unique geological feature. After more than 10 years of leading scuba tours through underwater caves around Hawaii, Chuck discovered some caves on land that he wanted to show visitors. When the land surrounding the largest cave on Maui went on the market in 1996, Chuck snapped it up and started his own tour company. His 75-minute walking tour ($29; no children under 8) is a fun, safe, and easy stroll through a huge, extinct lava tube with 40-foot ceilings. This is a geology lesson you won't soon forget -- Chuck is a longtime student of the science of volcano-speleology and can discuss every little formation in the cave, how it came to be, and what its purpose is. He supplies all the equipment you'll need: lights, hard hats, gloves, and water bottles. For those looking for a longer experience, his "Wild Adventure Tours" are 2 1/2 hours and cost $69 (no one under 15). Wear long pants and closed shoes, and bring your camera.

If you want to combine caving with a tour of Hana, contact Temptation Tours (tel. 808/877-8888). Their "Cave Quest" tour offers not only a 2-hour cave tour, but also an air-conditioned van tour from your hotel to Hana; the $169 cost includes continental breakfast, beachside picnic lunch, and a stop for a swim.