MIKE HUGHES - GANNETT NEWS SERVICE

When "Survivor" returns Thursday it will be rushing into some new worlds.
There's the geographic world of the Amazon River in South America. "Certain places are just very evocative," says Joe Ramirez, a travel coordinator who persuaded the show to go there.

Mostly, the Amazon evokes images of crocodiles, piranhas and dark secrets. In the opener (8-9:30 p.m. Thursday on CBS), the 16 contestants are taken to the river's Brazilian rain forest.

"Survivor: The Amazon," the sixth installment of the popular reality contest, strands 16 competitors in a remote location, voting one person out each week. The last person standing collects a $1 million prize. After Thursday's opener, the show will air every Thursday night at 9.

Going to the Amazon was as much a shock for the host as it was for the contestants. "I'd never been to the Amazon," says Jeff Probst, the show's host. "All I knew about it was from (reading) 'Heart of Darkness.' "

Then there was the psychological world of men versus women. That's new turf, too.

For the first time, the show begins by putting the eight men in one tribe and the eight women in the other. That required some quick strategy switches.

Some men, says producer Mark Burnett, had planned to use their brawn. That would be crucial in the competitions and the tribes would be reluctant to vote them out.

Instead, he says, they found games requiring agility. "These guys looked like they've been pumping iron for 20 years, (but they) couldn't get across the balance beam."

And some women, Probst says, had counted on their sex appeal to keep from being voted out.

Suddenly, that wouldn't work.

"They talked about that in the first day or two," he says, "that this is not the place you want to put on a string bikini to show off your ... tight body. But if we ever merge and there are guys around, clothes are coming off."

That's been the "Survivor" story -- constant shifts. Burnett has startled competitors by having them switch tribes; he's also used sharply different settings.

The first "Survivor" was set in a tropical paradise, but the next two -- Australia's Outback and Africa -- were harsh and unrelenting.

"It was so stifling," Probst recalls of Africa. "I was miserable -- and I had a tent to go back to."

Viewers were miserable, too. They didn't enjoy seeing stagnant, sweaty people who had little chance to swim or fish; ratings dropped.

After almost choosing the Middle East, the show switched tone after the Sept. 11 attacks. It went to the South Pacific and then Thailand.

"I just like water," says Probst, who would like to see "Survivor" try Iceland or Greenland. As it happened, Ramirez was trying to persuade producers to select one of the world's most awesome water sources as the next "Survivor" locale.

About 3,900 miles long, the Amazon River is the largest river in the world in volume. It is second only to the Nile in length. It's often so wide that people can't see both sides at once. Near its delta, one of its islands (Marajo) is larger than Rhode Island.

"It's incredibly large," says Ramirez. "It has something like 10 percent of the world's rain forest. ... There are places where there are no signs of civilization."

This is not like anywhere else the show has been, Burnett says. "It's not even on that planet. It's really a wonderful place, 2.5 million miles of the greatest rain forest on earth."

Ramirez is vice president of a small Miami firm (the Valorem Group) that links corporations and individuals with out-of-the-mainstream retreats. One of its clients is the Ariau Amazon Towers, which is near the spot (at Manaus, Brazil) where the Rio Negro river reaches the Amazon.

The Towers -- an air-conditioned, 300-room lodge -- has housed Jimmy Carter, Kevin Costner, Charleton Heston, royal families and now Probst, Burnett and "Survivor" staffers. Still, Ramirez says, the setting has some surprises.

One is the look of the Rio Negro. "The water is completely black," he says. "But it's considered to be the cleanest water in the world."

Another is the proximity of crocodiles and more. "The Rio Negro is teeming with piranha," Ramirez says. "But the natives swim in it."

Eventually, the "Survivor" contestants swam, too. "They were scared to death, didn't want to go near the water," Burnett says. "But a third of the way through ... they're swimming."

Their hunting was limited to a few ratlike critters, but their fishing was easier.

"There were tons and tons of fish," Burnett says. "Assuming the fish you catch don't bite off your finger."

Castaways for the CBS television series "Survivor: The Amazon," Christy Smith (back row, left to right), Matthew Von Ertfelda, Daniel Lue, Janet Koth, Butch Lockley, Deena Bennett, JoAnna Ward, Alex Bell, Heidi Strobel and Dave Johnson. Also castaways, Jenna Morasca (front row, left to right), Ryan Aiken, Rob Cesternino, Jeanne Hebert, Roger Sexton and Shawna Mitchell.