VOLCANO, Hawai'i — About the time jagged edges started poking through my T-shirt as I wormed my way through a tight spot in a lava tube, I began to wonder: What am I doing here?

We could have been relaxing on the famous Green Sand Beach at South Point of the Big Island. But in the spirit of adventure, we were instead exploring the world underground.

I had convinced my husband that it would be neat to go hiking to see flowing lava at Kilauea Volcano on the first night of our weekend getaway, and then get up early the next morning to go caving in 1,000-year-old lava tubes.

He was agreeable, even after twisting his ankle on the initial hiking excursion, which lasted past midnight. Once we got back to our cabin at Volcanoes National Park, we understood why each room had a Jacuzzi.

But we weren't about to let our sore muscles get the best of us. Watching the orange glow of waxy-looking lava hiss into the ocean had intoxicated us. We wanted to see tunnels formed by the molten stuff.

So we caught just enough sleep to wake at the crack of dawn and drive to South Point in search of a labyrinth of lava tubes at a place called Kula Kai Caverns.

To get there, we punched in a pass code and entered a remote neighborhood built on lava rock. Then we saw our guide's landmark, a thatched hut that looked like something out of "Gilligan's Island."

Our tour guide, Kathlyn Richardson, led us into the yurt and handed us spelunking helmets, lights, gloves and knee pads to gear up. We looked like coal miners without the soot, and we were about to go on a two-hour spelunking tour. So she tested us a little. Would we like to try shimmying up parts of the cave and crawling around in tunnels?

We were game.

Let me just say my previous experience in caves was limited to a guided tour years earlier on a heavily traveled pathway at Kentucky's Mammoth Cave. While it is considered the granddaddy of American caves — and the longest cave in the world — I saw only the easy-access "tourist" part of it. No shimmying through narrow passageways was involved.

And I'd seen Thurston Lava Tube, a major attraction on the drive around Crater Rim Drive at Hawai'i Volcanoes National Park. But that was just a peek into a cavelike shell.

I'd never seen much of caves and never could explain the difference between stalactites and stalagmites. Somehow, the idea of exploring a pitch-black volcanic cavern still appealed to me.

And that's just the kind of tourist Kula Kai Cavern founder Ric Elhard wants to educate.

"I started crawling around in caves when I was 12 years old," said Elhard, a California native who bought property on the Big Island because he knew there were caves underneath it. He and other cavers have since mapped out miles of lava tubes that crisscross the area.

"We're not doing big numbers, but we saw huge potential," Elhard said. "We want to be more of an educational tour. We want people to learn and understand about the archaeological aspects of caves."

He and other guides lead small groups, even with children as young as 5, through parts of the caves on tours that range from easy half-hour strolls to more challenging half-day explorations. When my husband and I were there, it was just the two of us, following Richardson into a braided maze she knew by heart and getting a taste of massive chambers and small spaces formed by volcanic gases rising through cooling magma 1,000 years ago.

A National Park tour, it is not. It's a fledgling operation that's loosely organized, and sometimes the battery-powered lights on the well-used equipment go out. But it's definitely an adventure tour.

We broke a sweat, despite the 68-degree temperature inside the cave, and had to watch our footing navigating craggy rocks that seemed to grow out from above and below. One of the most interesting parts was when we turned off our head lamps and sat in the dark, listening closely to dripping water and the sound of our breathing.

Squirming around on my hands and knees gave me a feeling not just for overcoming claustrophobia but for looking at a volcanic island from a different perspective.

I thought about the cave dwellers who must have used the spaces for shelter. I wondered how long the kukui nuts, which contained oil that Hawaiians used to light like candles, had been left on the rock shelves we passed. I pictured lava flowing through the walls I could now touch.

The tour captured my imagination. Yes, I could have spent the day relaxing on the beach. What I did instead made me feel like an explorer. Discovering a world I'd never seen before made the vacation feel complete.