BOB ABERNETHY, anchor: Now, the obsessions of Michael Fay. He is a conservationist with a single-minded passion to explore and protect the last wild places on earth.

Fay is an American, but for 23 years -- most of his adult life -- he has lived in central Africa, studying its ecology and helping create and manage a national park.

Fay flew over vast tracts of wild African forests and came to believe they were mortally threatened by logging and mining. So as a scientist -- he earned his doctorate studying gorillas -- Fay decided to measure that encroachment.

For 15 months, beginning in late 1999 in northeastern Congo, Fay led a dangerous, 2,000-mile trek through central Africa's most remote forests, ending up at the Atlantic Ocean, in Gabon. His almost religious objective was and is to create national parks to protect the wild forests he mapped. Fay thought he could best protect these forests by documenting exactly how human influence affects them.

He got the backing of the Wildlife Conservation Society and the National Geographic Society. He hired 10 Africans to carry supplies. He assembled high-tech equipment for geopositioning and digital recording. Then he set out to note every pile of elephant dung, every variety of large tree, every chimpanzee cry -- every animal. On the way, there were 100 encounters with elephants.

Another day, chimpanzees. Fay said he and his team were the first humans these chimps had ever seen.

With this gorilla encounter, each side seemed to be waving to the other. And always, every day, creatures few people have ever seen.

The food they carried and cooked was salted and freeze-dried fish and chicken. Also, cassava flour to make manioc paste. Fay and his men were resupplied by air drops once a month, and were only rarely hungry. But the work was so strenuous, Fay lost 40 pounds.

Fay was the absolute leader, responsible for everything -- directions, discipline, and medical care. This man got an infection, and Fay had him evacuated. No one died.

Walking in swamps and elephant footprints, all the men got worms in their feet. The standard treatment was disinfectant and duct tape. Other pests, such as ants, just had to be endured.

J. MICHAEL FAY: They get on you by the hundreds and bite you. It's no fun at all.

ABERNETHY: In spite of pests and swamps and thick brush, Fay's preferred wardrobe was always shorts and sandals.

One day, Fay recorded his thoughts as he came down with malaria.

FAY: Pretty feverish today, but got to make a few more Ks, got to put on a show. But we'll make it.

ABERNETHY: Fay huddled by the fire and recovered.

Once, on a shallow lake, Michael Nichols, a NATIONAL GEOGRAPHIC photographer, quietly approached an elephant.

The more untouched nature Fay saw, the more angry he got at signs of human encroachment -- such as the remains of a logging camp, the forest clear-cut all around it.

Photographer Nichols, an old friend, said the trip made Fay more of a conservation fanatic than ever.

MICHAEL NICHOLS: He is a super-radical now about the forest being a sanctuary, a temple; about it being preserved, left alone.

ABERNETHY: Four hundred and fifty-six days after Fay and his men began their trek, they emerged, at last, onto the seacoast of Gabon. They swam, and shared the water with surfing hippos.

FAY: Ah, la, la. Can't say I don't have tears to my eyes. That's for sure. Just like a total religious experience, no other way to describe it.

ABERNETHY: This year, after Fay returned to Washington, D.C., it became clear how much the trek had changed him. He says he was shell-shocked by the noise, stress, and waste of American city life.