The travel was rugged but fantastic, Damon says—moving from school to clinic, from one remote village to another. Damon's brother, Kyle, a sculptor who traveled with him on that first trip, has told me that the two were determined to downplay the star's fame, to be tough travelers and good students. But even the best of travelers can sometimes be undone by creepy crawlers in the night. Once, in a Zambian village, Kyle said, he and his brother "were outed as complete wusses." Confronted by giant bugs—prehistoric is the word Kyle used—the two of them hid under their mosquito nets and yelled for the bodyguard to come save them. "It was hard to view ourselves as tough guys, cowering under the net and clutching our malaria meds," Kyle says, with a typical Damon laugh.

After meeting the Zambian girl on the same trip, Damon became convinced that water was the issue to tackle. Ignoring his coffee, Damon leans forward and gives me the facts. Dirty water is the leading cause of cholera. Some two billion people worldwide live on less than two dollars a day. A child dies every 15 seconds because of dirty water. "The world water crisis is one of the most important public health issues of our time," Damon says. "Clean water can help put people on the first rung of the development ladder."

Damon and a team of friends hatched the idea of the H2O Africa Foundation, which partners with Jeffrey Sachs's Millennium Villages Project to deliver funds for water wells, as well as with OneXOne, a foundation that supports children around the world; Ryan's Well, which provides water and sanitation education; A Glimmer of Hope Foundation; Living Water International, which provides wells for refugee communities in the Central African Republic; and the One Campaign, a grassroots U.S. organization that tries to pressure Washington to increase global aid.

Damon's philanthropic impulse has its roots in the commune-like group of six families of former hippies in which he grew up in Cambridge, Massachusetts. His values were instilled in him by his mother, a children's education specialist, who remains his moral compass. Asked to do a voiceover for a bank commercial years ago, he turned to her to discuss the ethics. Was it okay to take money from a bank? Yes—if he gave it all to great causes. "I thought it would be like that Paul Newman motto," he adds with a laugh, referring to Newman's Own, which gives its profits away to educational and charitable programs: "?'Shameless exploitation for the common good.'?"

Devoting significant time to philanthropy had to wait, however. "It's such a mountain to climb to get a career going in the entertainment industry, so it was always in the future," Damon says.

The stars aligned a few years ago around Running the Sahara, a documentary about three marathoners running across the Sahara through such water-starved countries as Senegal, Mauritania, Mali, and Libya. Damon decided that the film, which will be released this month, would be a perfect vehicle to highlight the problems of Africa.