Marine scientists sailing the central Pacific to study its remote coral reefs have reached an underwater Eden even more pristine than they had hoped for, according to e-mail dispatches they sent Sunday.

The elaborate coral structures of Kingman Reef, an atoll about halfway between Hawaii and Australia, are teeming with tiger sharks, dolphins, aggressive snapper, green turtles and giant manta rays -- as densely packed with sea life as a fully stocked tropical aquarium.

"Eureka!" expedition leader Enric Sala wrote in an e-mail sent by satellite from the White Holly, which is based in San Francisco. "We have found it -- a pristine reef, where corals are alive and healthy and form a forest so thick that there is no space even for sand between them."

In contrast, the scientists found clear evidence of overfishing and global warming at three nearby islands: Tabuaeran (population 1,500), Kiritimati (population 10,000) and Palmyra atoll, which was briefly occupied by the United States during World War II. The scientists will next visit Teraina Island (population 950) in the Republic of Kiribati.

"Coral reefs are the canary in the coal mine for our planet," coral expert David Obura of Kenya wrote via e-mail. "The trip is showing, along with the work of my colleagues worldwide, that the planet is suffering from man's thirst for more and more wealth, products and energy."

The 133-foot White Holly, a former Coast Guard buoy tender, anchored Thursday for four days in the sheltered lagoon of Kingman Reef, a boomerang- shaped atoll in the Line Islands archipelago near the equator. Annexed by the United States in 1922, the reef is a wildlife refuge that has experienced little if any fishing since World War II. Seas there are usually so rough that ship captains tend to steer clear.

Two dozen of the world's leading specialists on coral reefs and fisheries are participating in the expedition, which is led by UC San Diego's Scripps Institution and funded by a grant from the Bay Area's Moore Family Foundation. They conducted numerous dives to explore the reefs -- counting, measuring and photographing myriad species of fish, algae and invertebrates and collecting coral and water samples for microbial and chemical analysis.

"We were not sure if we were dreaming," wrote Sala, a Scripps marine ecologist and conservation biologist. "Fifteen meters below our boat was a dense forest of living staghorn and table corals, filling all of the bottom."

The scientists hope to learn why so many coral reefs are dying and to quantify the characteristics of healthy reef systems in order to set goals in restoring coral reefs elsewhere.

Participating are researchers from Kenya, Australia and the United States, including marine biologist Jen Smith, a graduate of Drake High School in Marin County. Smith, an expert on algal taxonomy and coral reef degradation, is affiliated with the University of Hawaii and the National Center for Ecological Analysis and Synthesis at UC Santa Barbara.

"We jumped in the water and tens of sharks and large snappers immediately swam up to the boat to investigate us," Sala wrote. "After giving it some thought, we decided to jump all together. We did, the sharks swam away, and they came back later with a less aggressive behavior. Safety is in numbers. ... These were fishes that have never seen humans before."

Pollution, disease, overfishing and rising ocean temperatures have been linked to the global decline of coral reefs. Conservationists say that roughly one-quarter of the world's warm-water coral reefs have disappeared and that two-thirds are at risk.