The mountains rise, silent and brooding over western Maui. Dark, green mountains, covered with a seemingly impenetrable wilderness, they suggest a primeval landscape. Almost 6,000 feet at their highest point, the West Maui Mountains are the remnant of a single, centuries-old volcano that has weathered into deep valleys and inaccessible gorges, vast craters and soggy marshes, knife-edged ridges and rushing streams. Most days, mists hug the valley floors, and clouds shroud the mountaintops. The region harbors a diversity of spectacular topography, from Violet Lake to ¿Eke Crater, a near-perfect circle of lava, the heart of the ancient volcano.

Surprisingly, it is not stone but water that characterizes these little known peaks and valleys. An annual rainfall of close to 400 inches has helped carpet the mountain with a lush rain forest. And absorbed in, sloshing around, and trapped under the mountains are millions upon millions of gallons of pure, fresh water that some call the sweetest in the world. The West Maui Mountains are like a giant sponge.

Most of the mountains' upper reaches are restricted: People are not generally allowed in, and if they knew how wet and inhospitable the area really is, few would want to go there anyway. Occasionally, ranchers will search for a lost cow, or an intrepid hunter will go after a wild pig. But for most folks here, the mountains are a strange, impassable complex of crags, bogs, and rain forest.

They are also a dense, green home for some of the rarest animals and plants in the Pacific. And so, drawn by that fact, I find myself riding up a pot-holed, boulder-strewn jeep road with a couple of friends, following the south rift of the original West Maui volcano.

Bob Hobdy is a tall, big-boned, gray-bearded naturalist with a wide smile, an affable manner, and an encyclopedic knowledge of West Maui's flora and fauna. He grew up on the nearby island of Lanai and has been wandering Hawaii's wilderness areas since he was a small boy. Now, at 54, he works for the state's Division of Forestry and Wildlife and regularly out-hikes and out-climbs people half his age.

Mike Severns, wiry and bristling with energy, is a photographer, the owner of a dive operation in nearby Kihei, and a biologist with a particular fascination for mollusks. At 46, Severns has swung down into West Maui's chasms from a helicopter, poked into caves and lava tubes, discovered the bones of extinct birds (one of which is now named after him), and mapped out the distribution, taxonomy, and morphology of several species of endangered land snails.

Early on this Saturdaymorning we meet at a little general store at Ma¿alaea Harbor, wolf down hot dogs and Gatorade for breakfast, and then gather our boots and rain gear. Our plan? Drive to McGregor Point on Maui's leeward side, unlock a fence, drive five miles up into the mountains, and then walk.

At sea level, where we start, we are in the rain shadow of the mountains; the land around us is bone dry and covered with brown grass. It shades into the low scrub typical of Hawaii's coastal forests. Small wiliwili trees grow out of the sparse soils in the cracks of pre-historic lava flows.

Soon we pass windswept kiawe trees; then, above the open slope at 1,200 feet, near some rock outcroppings, we encounter the first native forest plants.

We find a¿ali¿i, a tough, compact shrub sacred to the traditional goddess of the hula. There are drooping, shiny-leafed ¿akia, some of which are poisonous, and pukiawe, used in chiefs' fumigation rituals. We come across dwarf ¿ohi¿a lehua, the beautiful red-blossomed tree associated with the volcano goddess, Pele. And, as we gain elevation, we see the more mature, denser ¿ohi¿a trees that characterize so many of Hawaii's wet forests.

We continue to jolt our way up the mountain, passing long, rolling swales of green grass and startling a huge owl out of his perch in a koa tree. When the owl flaps off in irritation, we realize we are above the heat and in the cool of the West Maui forest, where the temperature is ten degrees lower than at sea level, and the ground is damp. Large volcanic domes, jagged peaks, razorback ridges, and deep ravines surround us, and we can see fog hanging in one of the valleys below.

Finally, when the road ends, we park the vehicles, lace up our boots, and start to hike in.

Twenty paces up the skinny path stamped out by wild pigs and semiwild visiting naturalists, Hobdy stops and begins poking around in the underbrush. "Botanizing," he calls it - identifying plants and noting their location, range, health, and variation.

As we work our way through the forest, Hobdy points out delicate native ferns, a rare terrestrial orchid called plantanthera (only a dozen have been found), and various members of the primrose, sedge, and lobelia families, including a Lobelia hypoleuca with long, delicate, azure flowers.

Severns is no slouch when it comes to local plant lore, but his main interest focuses on a genus of endemic land snails called Partulina, small mollusks that have adapted to rain forest conditions. Severns is just as compulsive about snails as Hobdy is about plants, and they banter good-naturedly over whose specialty is more important.

"Flora," insists Hobdy, "is the basis of everything up here, including all those snails."

Severns purses his lips. "The problem with plants," he finally replies, "is they don't move. There's no behavior to watch."

In fact, both of their interests merge another 50 feet up the trail. Snail Man notices an old o¿hia tree, and Plant Man, inspecting the underside of the leaves, locates a colony of Partulina splendida, little dark brown snails with tan spiral bands.

After another mile the trail gains altitude. Every new ridge and valley in the West Maui Mountains seems to have its own kind of plants and animals. Along the side of one ravine we spot a crimson ¿apapane, a member of the Hawaiian honeycreeper family, and then find ourselves in what naturalists call an epiphytic vegetative zone.

Epiphytes are simply plants that live on other plants. Here in the extreme wet areas that make up the heart of Maui's cloud forest, soil is often lacking altogether. Instead, the trees themselves become hosts for dozens of species of vines, ferns, mosses, lichens, and liverworts. Some of these plants are extraordinarily fragile and primitive.

Hobdy points out a thin, filmy, epiphytic fern, named Vandenboschia davallioides, that is literally just one cell thick. It is so highly adapted to life in a moist environment that it dies if there is too much sunshine.

Even the insects that live in this zone depend on specific plants for nourishment and camouflage; they are incapable of living in other parts of the forest.

After several hours of exploring the ridges just below the summit of Hana¿ula, we begin our descent. At an overlook in the very back of Ukumehame Gulch, we spot the white tails of two pairs of tropicbirds soaring on the currents several thousand feet below us. Other discoveries lie closer at hand: mushrooms in hollow logs, gardenias and greenswords, bright pink lobelias, several more native orchids, and - to Mike Severns' delight - more snails.

We stop to pull out clumps of tibouchina, an invasive ornamental plant brought to Hawaii from Brazil. A sprout here or there seems innocuous enough, but left to its own, a single tibouchina plant can grow into a patch five acres wide and eight feet tall. It's now making its way into the heart of the West Maui Mountains, threatening to crowd out native species.

"Fifteen years from now," says Hobdy sadly, "there is a real likelihood that many of the magnificent native plants and animals that we are seeing will be gone."

Conservation and education efforts are under way, however. Hawaii has designated four separate West Maui Mountain tracts as natural area reserves. Some private landowners and firms, including the Maui Land & Pineapple Company and AMFAC, have forged partnerships with The Nature Conservancy in order to keep their lands pristine.

We squish our way through small bogs and muddy hollows, clamber over fallen trees, stop to root out another patch of tibouchina, examine more snails, and finally arrive back at the waiting vehicles. Slowly we bounce our way back down the rutted road.

As we approach sea level, the heat rushes through the windows. When we come to the fence, I get out of Hobdy's truck, unchain the barrier, and we drive through. Behind me, amid the glimmering mists and gathering clouds of the West Maui Mountains, I can just make out the faint hint of a rainbow.