Hawaii Volcanoes National Park, Hawaii (CNN)
Picture a volcano spewing a river of molten lava burning so hot that it burrows its way through the Earth, moving so fast that a human couldn't outrun it.
That's what happened on the Big Island of Hawaii at what is now Hawaii Volcanoes National Park.
The lava flow from the Kilauea volcano left behind massive lava tubes as evidence of its destructive path, scientists say.
While the Hawaiian Islands were created by 70 million years of volcanic activity that continues today, the lava tubes were created only about 500 years ago, they say.
In honor of the National Park Service's centennial celebration this year, which is also the park's 100th anniversary, Hawaii Volcanoes National Park is giving a few visitors the chance to take a guided tour of one pristine tube called Puapo'o.
The tube was discovered in 1991 when workers putting up pig fences deep in the forest happened upon it.
The park welcomed more than 1.8 million visitors in 2015, but only about 600 will get to venture inside Puapo'o in 2016.
Most of the lava tubes in the park are closed to the public to protect them. Puapo'o is hidden in the rainforest, far from the trails in the 520 square miles of the park.
Friends of Hawaii Volcanoes National Park, a nonprofit that supports the park service here, wants to keep it that way.
"It's so delicate in here, and it's so fragile," said Ab Kawainohoikala'i Valencia, a Friends tour guide giving CNN a tour. "We don't want people to just come in here and do things they shouldn't be doing."
'It's red hot in here'
The 4-mile hike to Puapo'o, which includes a 500-foot elevation change, takes some physical strength. Visitors are required to wear long-sleeved shirts and pants and are outfitted with gloves, hard hats and headlamps.
A flashlight is also helpful, because it's pitch black inside the cave.
After climbing down to the opening, the entrance is obscured by flora. Large chunks of fallen lava create the rocky path that visitors must find their way over and around throughout the mile-long journey.