Few members of the public have ever been allowed to visit this place. But after years of inquiries, I was invited to join a scientific expedition into the valley. Leading the trip is 42-year-old Seth Judge, a wildlife biologist from the University of Hawaii at Hilo. He sits beside me, snapping photos and taking videos of the forbidding terrain, where he has been conducting bird surveys for the past two years. “Look at this place,” Judge says through radio static and the percussive thwack of the blades. “It never gets old coming up here.”

It’s no accident that the Kīpahulu Valley is one of Hawaii’s last bastions of more or less pristine biodiversity. This massive erosive scar running down Haleakalā’s eastern flank is more than 750 meters (2,500 feet) deep in places. That challenging topography makes it an almost perfect natural fortress, protected by near-vertical cliffs and dense vegetation. The terrain has isolated the valley, keeping its ecosystems mostly intact while driving a stunning adaptive radiation of unique organisms. “A lot of species are endemic here because they were able to evolve without interference from species from the rest of the island,” says Woody Mallinson, Haleakalā’s natural resources program manager. Today, as many as 13 threatened and endangered species of animals and more than 40 species of plants inhabit its roughly 3,400 hectares (8,500 acres). Dozens are found nowhere else on Earth; dozens more have disappeared. “For some of the listed species, they are historic and haven’t been seen in decades,” Mallinson says.

The oft-told narrative of Hawaii’s ecosystems is one of continual assault by foreign invaders, including escaped pests like rats; domestic animals, such as pigs and goats; and crops like sugarcane, coconut palm, and taro. Even the supremely isolated Kīpahulu Valley is not entirely immune. To keep its assemblage of rare species protected, the National Park Service has erected a defensive perimeter of fences around it.