TOWARD NOON on day two, as Bruno Baert, the director of logistics for WCS Gabon, and I were attempting to force one of the vaches through yet another snag, we looked downriver to see one of the faster green boats signaling to us. Something was happening around the next bend, but by the time we caught up, it was all over.

The two lead canoes had noticed a great gray boulder set back in the high sawgrass. Suddenly the boulder had exploded in a full-on charge, right down to the edge of the river—a bull elephant with flared ears and magnificent tusks that had somehow escaped the poacher's knife. Then, with an imperious snort, he'd disappeared into the forest. The four had been just 20 feet away and were still reeling from an experience that, as Boyer put it, was "so real it seemed fake."

Baert and I groaned and resolved to paddle harder. But an hour later, the same thing happened—another elephant sighting, with us just out of range behind. No charge this time, the green-boaters assured us. Nothing special. Baert stood up. "I want to see an ay-lay-phant," he said. "I need to see an ay-lay-phant."

By the end of the day we'd covered five miles as the crow flies and nine over the ground—almost twice our first day's total but still not nearly enough, we thought, to squeeze the Djidji into a single week.

And yet at some point that evening my attitude changed. It helped that our campsite was perfect, an old poacher's camp on a promontory above the confluence of a small tributary. The bloodsucking tsetse flies that had plagued us all day had vanished, and the eerie daytime silence of the forest gave way to the amazing cacophony of the African night: the pounding chorus of a bat colony, the scream of the tree hyrax, and a distant timpani-like sound that Starkey and Curran said was a gorilla pounding its chest. The hissing fire; the soft grunts of Baert and Gnoundou as they hooked and landed perch, catfish, and the primitively scaled capitaine; the unbroken wildness beyond—this was what camping in the equatorial rainforest was meant to be.

Alex Tehrani, the photographer, must have been similarly inspired by that exquisite moment, because he jumped up, grabbed a tripod out of the tent we were sharing, and walked a few feet into the forest to take some long-exposure shots. A minute later I heard him call out sharply.

"Ow," he said. "Damn! Fuck!"

Then, a moment later, the one phrase I wanted to hear even less than "snake in the boat":

Then, a moment later, the one phrase I wanted to hear even less than "snake in the boat": "They're all over me!" Tehrani came sprinting back to the fire, slapping madly at the army ants on his legs.

"They're all over me!" Tehrani came sprinting back to the fire, clutching his camera and tripod in one hand and slapping madly at his legs with the other. "Army ants, also known as driver ants," Starkey said helpfully, before zipping himself into his own tent. Curran laughed. "Welcome to the Congo Basin," he said.

The bites hurt, but the sting didn't last. "They're not really a problem unless you're tied to a tree," Curran said. So once Tehrani had de-anted himself, we crept back to the edge of the clearing for another look. The column had by this time overrun our unzipped tent—there was nothing to do but pick it up, shake everything out of it, and then hold it over the fire and let the smoke drive out the last of the interlopers. A particularly gruesome tableau was formed by my muddy Tevas, which were completely encrusted by a wriggling layer of ants—apparently there was something in the river clay they had to have.

Later, lying in the tent, I could hear the army moving over the leaves of the forest floor—a million crinkly footsteps, like high-pitched rain. In the morning, we awoke to find my sandals picked clean and the ants long gone.