MAHA'ULEPU, Kaua'i — The Makauwahi Sinkhole, the largest limestone cave complex in the Hawaiian Islands, is yielding an unprecedented look into Hawai'i's history, with a record of life that dates back 10,000 years.

The findings from a multiyear archaeological dig at the sinkhole have profound implications for proposals to reforest parts of the archipelago with native vegetation, since it shows that coastal forests included a wide range of plants long thought to be limited to upland habitats.

The site also reveals a rich array of bird life, and has changed the current understanding of what pre-human Hawai'i looked like.

Several previously unknown bird species have been identified from fossil bones. The complex coastal forest of the region has been intricately described from seed and pollen remains. Two species of plants — kou and hala — that were once believed to be Polynesian introductions have been proven to predate human arrival. There are signs of ancient snails, extinct land crabs and much more.

The impact from the arrival of the first humans in the Islands also is immediately visible in the cave's sediments. There are the bones of rats, which traveled with voyaging Polynesians, and evidence of the immediate collapse of plant species on which rats fed, such as loulu palms. Later there are fishhooks, pieces of outrigger canoes, charcoal from early imu, and other artifacts such as stone tools and a round basalt mirror. And there are human burials, which are being carefully preserved in place.

"You've got this continuous record, like a slow movie through the 10,000 years," said David Burney, a Fordham University professor and director of conservation at the National Tropical Botanical Garden on Kaua'i. Burney, an ecologist and archaeologist, and his wife, Lida Pigott Burney, also are overseeing the reforestation of the cave and its environment with the plants that the evidence proves once forested the region. A partner from the earliest days of the project was the late Kaua'i archaeologist Bill "Pila" Kikuchi.

Pollen and seed found in the pre-human layers prove many plants that are now rare or only known from isolated or upland areas were once part of a complex coastal forest. That led Burney to the theory that today's dominant native forest trees like 'ohi'a, hala and koa were once comparatively small parts of the forest ecosystem. Once the ecosystem was disturbed by human contact, these survivors were able to gain greater density as other plants disappeared.

"Probably the key difference between pre-human times and now is that the early forests were much more diverse," Burney said.

The site also has turned out to be the richest fossil site in the Hawaiian Islands. Burney and his fellow diggers have found the bones of at least 45 species of birds. There are strange creatures, such as turkey-sized flightless ducks and a tiny duck with eyes near the back of its head that may have been a night-feeding creature with some characteristics of New Zealand's kiwi. There are the remains of owls, a bat, a gull and several forest birds that are all now extinct.

SITE OF THE DIGS

The pale rock ridge that houses the sinkhole started as a field of sand dunes. Over time, rainwater seeped through the sand, converting it chemically into limestone rock. Underground water ate away at the lower parts of the limestone, forming an extensive complex of caves, and finally one large section of cave roof collapsed, creating a feature known as a sinkhole. The feature is as much as 100 yards long from the entrance to the most distant known cave, and as much as 40 yards wide, but it may contain other caverns whose entrances are buried.

Plants and animals have grown and died inside the cave. Birds have fallen in or become prey to owls that roosted in the cave. Pollen, seeds, leaves and branches have fallen in. Each layer of once-living material was covered by dust and sand, forming the base for another layer.

Burney bored holes into the cave and found that the layers of material — going down as far as 30 feet to the rocks that once formed the top of the cave — were amazingly well-preserved. There may be sediments older than 10,000 years below the rocks of the collapsed cave roof, but he has not yet figured out how to get through the rocks.

"One of the good things about this site, other than the excellent stratification (layering), is the preservation of perishable materials. This is like a giant pickling jar. Leaves, whole extinct land snails, bird, seeds, fish with scales still on — they are all remarkably preserved," said Burney, who has done cave research in Africa and on Madagascar and Puerto Rico.

LAKE BOTTOM

Over time, the site has sometimes been a lake of fresh or brackish water, and much of the material has been preserved because it was laid down on the lake bottom, protected in a low-oxygen environment.

Working with students and other volunteers, Burney followed up bore holes with a series of pit excavations, carefully peeling back the layers of time with the layers of sediment. During the digs, pumps are run constantly to keep groundwater from refilling the excavations.

In the modern period, the diggers find a lot of plastic, and going back, there are iron nails, then bronze and iron nails bent into fishhooks, then nearly 1,000 years of Hawaiian history, and before that, the pre-human period with its unexpected revelations of forest composition and strange animals.

"In the Hawaiian period, we found a lot of rope and string. Some of it is (fiber from the plant known as) olona. You can see the kind of braid they used and the knots they tied."

Burney said his goal is not only to understand the past of the region, but to use it to build a future for Makauwahi, now flanked by a golf course, a limestone quarry, abandoned canefields and the sea.

Burney and his wife have a license to the immediate site and surrounding areas from landowner Grove Farm Co. At the company's request, they put an iron gate at the low crawlspace that forms the cave entrance to prevent tampering with geological features, plants, archaeological digs and burials. The public is welcome to visit the site when someone is present.

RE-CREATING LANDSCAPE

One goal is to use what they've learned from their digging to re-create a landscape the first Polynesians may have encountered when they arrived.

As an official of the National Tropical Botanical Garden, Burney has access to many rare plants, which has been useful in the reforestation of the sinkhole floor and the planting of the region around the site.

Burney said he hopes to re-create the original wetland that once adjoined the cave system and to replant former cane fields in the area with native forest.

Along with the expected coastal plants, they are planting native grasses; naio or false sandalwood; the Kaua'i tree cotton Kokia kauaiensis and a range of other hibiscus relatives; and 'a'ali'i, kou, hala, koai'a, maiapilo, 'akia, 'akoko, aulu, ko'oko'olau, wiliwili and more.

When they sought to determine which species of native fan palm to grow, they studied preserved seeds and identified them as very much like the Ni'ihau variety, Pritchardia aylmer-robinsonii, which also grows in low, dry environments.

There are believed to be only two specimens of these palms left in the wild on Ni'ihau, but no one knows how widely they once grew and whether they once existed on Kaua'i.

When planted inside the sinkhole, the Ni'ihau palms burst upward, feeding on the rich sediments and drinking from the shallow water table.

As if they belonged.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.