When bus drops us next to a rust-colored dirt runway, we re-pack our luggage into single-engine airplanes. Tom looks like the political consultant James Carville on a fishing trip: sharp nose, dark piercing eyes, shiny head. His earthtone state-of-the-art hiking clothes blend-in.

From the air, I get a sense of the vastness of the Pantanal. 90-thousand square miles of flood plain stretching into Bolivia and Paraguay. Muddy rivers twist and turn back on themselves through the thick, green forests and open grasslands. Sky-blue lakes dot the landscape.

The "Pantanal" is the world's largest flood plain, about as big as Iowa. It's really only flooded during the rainy months, October through March. When we visit - in August - no rain falls.

We touch down on a grassy landing strip at an old cattle ranch - the Fazenda Rio Negro. Conservation International bought all 20-thousand acres in 1999, and now dedicates it to scientific research. A few cows still graze in front of the main house, alongside a handful of rheas - South American cousins to the ostrich.

The sound of hyacinth macaws greet us from high in a palm tree. Their brilliant indigo feathers point toward the red-tiled roof and white walls of the ranch house. Tom and I lug our bags further on, towards the stables and the Rio Negro river, to our home for the next 12 days. Small, spartan rooms. Four beds, a ceiling fan, and a bathroom. Good digs, says our new roommate.

Field note for: Rob Roberto. A San Diego probation officer. Muscle-mad body-builder. Public library volunteer. And Earthwatch fanatic.

Rob: "I use up all my vacation on Earthwatch...I started four years ago was the first kind of extensive travel I've ever done. There was always financial considerations. Quite frankly, I never had the resources to travel."

After a big meal that night, the lights dim for a slide show: a preview of coming animal attractions. Doing voice-over - Don and Alexine. Husband and wife scientists. Small, curly-haired Alexine traps and studies peccaries, a distant cousin of the feral pig. While helping her hunt for them, she says, we might see giant ant-eaters, the horse-like Tapir, and maybe even...

Alexine: "This is a jaguar track... You usually see their tracks, but every once in a while you're lucky enough to see one."

Don: "This is another detritus-eating fish. I have sort of a thing for scum-eating fishes...The piranha, their mode of living is to take chunks out of other fish."

My first job involves no data collection. Tom and I pull duty cutting trail. We hack at the subtropical forest with machetes.

Tom: "We're topiary school flunk-outs. Ha-ha."

Next day, we wake to the sound of the chachu-chachalaka birds singing back and forth to each other. Around 8 am, Tom and I set out with Alexine to hunt for the pig-like peccary. That means setting traps.

Tom: "Do you have a welder on the farm?"

Alexine: "Yeah. Why? Do you want to weld something?"

Tom: "Well, if you put these bars across the top you could cut your time in half for setting them up."

Alexine: "I know. Do YOU know how to weld?"

Tom: "Sure."

Around noon, we start the hour drive back home for lunch. Tom - the pragmatist - can't help but think how much time we'd save by packing a few sandwiches and eating in the field.

But the group meals on the back patio give me a chance to continue my studies. Turns out, practically all these folks have done this working-vacation-thing before. I sit with Harvey Lowe, a New Jersey financial analyst; Lada Kradkey, who writes children's books and teaches in Carmel, California; and Paula Atkeson, a psychoanalyst from Washington, DC.

Harvey: "I get that same question also. WHY? Why are you going there?"

Lada: "There are types of people who like to sit at a beach. It would drive me INSANE."

Harvey: "I work with a guy. They've gone to the same beach for 11 straight years. OK, just put a gun to my head and shoot me."

Paula: "Working in the environmental field wasn't an option when I was choosing a career. It's wasn't a field...So this is a way to break into a new field without having to go get another PhD."

That evening, my room-mate Rob gets back after dark. He's euphoric. His group had seen not one, but TWO jaguar. On the river bank, not 20 feet from their boat.

Rob: "And the behaviors involved, being the mating season. The female constantly enticing the male. Rubbing up against him, and rolling on the ground next to him, and using this kind of growl-purr type of vocalization. So it was quite amazing. It will be the highlight of the trip, no doubt about it. It's something I'll probably remember for the rest of my life."

Under the light of the full moon, Harvey Lowe, myself, and a couple of ranch hands take a boat out. A grizzled old cowboy sits in the bow, rubbing his fingers inside a wooden drum to replicate the cat's call.

We drift down river for three hours. But get no reply. And I realize that searching for animals can be tedious. I begin to see a new genius in the Earthwatch program: It puts you in the right environment to see wildlife, and then occupies your time until animals show up.

About half-way through the expedition, Tom and I go up-river with Don Eaton to take samples from a small lake. We pass caiman sunning themselves on the banks. These mini-crocodiles, about three feet long, watch us with big, black, unblinking eyes.

The aquatic survey turns out to be much more fun than I expected. Working with Don is like being a Boy Scout.

Tom: "Good bug buzzing by in head...ok, 8.51...Temperature is 27.7 degrees Celsius."

Jeff: "Does it seem at all weird to wade out here with piranhas and caiman?"

Tom: "You sort of try to put it out of your mind...what could go wrong?"

Tom: "Look at it. Oh my god! Oh man, look at that. The piranha took half that fish away...That was bizarre. Did you see? The net just started vibrating. It was literally buzzing with the energy of these fish."

Back at the homestead, I sit with Tom on the patio to gather some more data of my own. The question is crucial: as a first-time Earthwatch volunteer, does the trip meet expectations? After all the sweat equity you've invested, has it paid off?

Tom: "I'm more than satisfied with the wildlife I've seen. Every opportunity I've taken to see wildlife, I've just seen copious amounts. If I had any objections at all, I think they are underutilizing the total skills of the people here. It's not to say this isn't good. I think it's an excellent start to get people coming here and contributing both financially and with their own sweat. But of course, it could be improved, like everything else."

Jeff: "A true engineer..."

Tom: "Shoot the engineer, start production. That's our motto."

So I ask Don Eaton...Are these untrained tourists really that useful?

Don: "We certainly need the funding they provide. That's very, very important. This place wouldn't run without that. However, it's very easy to use a bunch of willing people to collect lots of important information quickly. They come here ready to go. Whereas say you dragged a grad-student here, they wouldn't really be into it."

During the course of my research, I've concluded that Earthwatch travel is similar: The scientists get funding and free labor, which helped Alexine catch a peccary. And the volunteers get animal close-encounters and a sense of contributing to a greater cause.

Tom Griebe is hooked. He's thinking of coming back to the Pantanal next year to study birds.

Rob Roberto can't wait. He will fly from Brazil to Wyoming, to work on ANOTHER scientific project. This time, studying bats with The Nature Conservancy.

Rob: "The problem with that is that I'll have double-depression, because I'll be considering BOTH trips when I go back to work."