The largest feature of the Earth is the mid-ocean ridge, a chain of mountains running along the floors of the Atlantic and Pacific oceans and covering almost 30 percent of the globe. The mid-ocean ridge is the most unexplored region on earth -- man has seen only 40 miles of its 40,000-mile length.

At 8 o'clock tonight on Channel 26, PBS' "Dive to the Edge of Creation," a National Geographic special, takes viewers on a short walk through some of those 40 miles as its cameras journey along the Galapagos Rift, a rip in the ridge west of Ecuador that the research submarine Alvin explored early last year. In a one-hour documentary, geologist Robert Ballard and biologist Fred Grasse recreate their dives in Alvin almost two miles down to the bottom of the Pacific.

In the Galapagos Rift, the Earth is still creating crust. Hot lava is pouring out through cracks in the ridge and cooling on the bottom of the Pacific. Water as hot as 800 degrees comes out of the same cracks, creating some of the warmest ocean anywhere on Earth.

"This is why we picked the Galapagos Rift for our expedition," Ballard explained on a recent visit to Washington. "We had some heat-flow data that suggested there was hot water coming out of the Earth at the Galapagos and we wanted to find out why."

An underwater geologist who works at the Woods Hole Oceanographic Institution, Ballard first dove to the Galapagos Rift in 1977. He went alone, never expecting what he found: The ocean floor at the Rift was teeming with life, even though no sunlight penetrated that far down. Crabs, clams, mussels, octopi, sea spiders and rat-tail fish were living on the Rift by the millions.

Ballard found a riot of color along the darkened floor.Bright yellow organisms bloomed like fields of dandelions.Giant red-tipped worms living in tubes 10 feet long were so numerous Ballard named their nesting place the Rose Garden. Ballard gives other names to places along the floor: the Dandelion Patch, the Oyster Bed, the Garden of Eden.

Ballard and the Alvin went back to the Rift early last year, this time with a new underwater camera and Fred Grasse to document the oasis he'd found two years before. In the film, as the Alvin moves along the Rift, Ballard turns to Grasse and exclaims: "You don't see this much life in shallow water." Grasse comments: "The hard part is going to be figuring out how these animals got here in the first place."

The animals they find along the sunless Rift floor are like no others on Earth. The crabs are a newly discovered family, sightless and white. The mussels are a previously unknown species. The clams are red; the tube worm is the largest anybody has ever seen. The "dandelions" are fragile, multicelled creatures related to the Portuguese man-of-war.

The source of food for the Rift creatures is the bacteria that live on the sulphur pouring out through cracks in the Rift and thrive in the warm water. Water samples taken by Ballard and Grasse reveal that life on the Rift is created by chemosyntheses instead of the photosynthesis most of the Earth's plant life uses to grow.

Ballard is just as interested in the mineral beds he finds along the Rift. He points out magnesium, manganese, lead, copper and zinc deposits as the Alvin drifts along the Pacific floor. Ballard says these metals are in abundance all along the mid-ocean ridge where the Earth's crust is constantly falling apart and tearing giant cracks in the ocean floor. Seawater seeping down through the cracks gets heated up inside the Earth, then carries the minerals back with it when it recirculates out of the same cracks.

"We now believe the entire volume of the ocean goes inside the Earth and out every 8 million years," Ballard said. "It goes down, it's heated up and changes its chemistry, and reappears very quickly. It doesn't go down and wait for 8 million years, but it's doing so much of it that the volume turnover is the whole ocean in 8 million years."

Ballard's got plans that go beyond the Rift and one gets the feeling we'll be seeing him, Fred Grasse and the Alvin again. Ballard wants to journey along the mid-Pacific rise near Easter Island, far to the west of Galapagos, where the sea floor is spreading three times faster and where underwater volcanoes are heating the bottom of the Pacific to even higher temperatures than Ballard found at the Rift.

"We don't expect it to be that much hotter, but we do expect it to be more pervasive," Ballard says. "That means the mineral deposits might be more extensive and the life more bizarre, but of course we can't be sure. That's why we want to see for ourselves."