THE TENTS WERE glowing like night lights as I wandered among them, desperately looking for our expedition medic, Melissa Arnot, and her stash of Imodium. It was 12:30 A.M., 15,000 feet up on Tanzania's Mount Kilimanjaro, but everyone was awake and about, getting ready for the start of our summit push. I ducked past the 12-person film crew and into our meal tent, where Jessica Biel, Emile Hirsch, and Lupe Fiasco were sipping tea and peppering Arnot with questions.

"What will be the hardest thing?" Biel asked.

"It'll get harder to breathe. It feels like there is a hand pushing down on you," replied Arnot, who explained a technique called pressure breathing that forces more air into your lungs.

"This is the hardest physical challenge I've ever done," Biel said. "I am not turning around now."

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This nylon city was part of the massive Summit on the Summit expedition, which had been dreamed up by the Ethiopian-born musician Kenna to bring attention to the global water crisis—a worldwide shortage of clean drinking water that kills more children (some two million per year) than any other single cause. Meanwhile, in the U.S., seven billion gallons per day soak into the ground thanks to leaky pipes.

It's a big issue, but it's not one that ranks particularly high in the American consciousness. Biel, Hirsch, and Fiasco, along with a dozen other experts and activists, were there to fix that. I was there to write blog posts, chosen because I'm a screenwriter and know when to keep my mouth shut, unlike the dozens of reporters who'd tried to get on the trip.