In 2007, fishermen living along the lower Congo, the deepest river on Earth, brought Melanie Stiassny a fish. It was six inches long, ghostly white and eyeless. Like most fish held out of water, it was dying. What surprised her was what was killing it.

“There were nitrogen bubbles forming under its skin and gills,” said Dr. Stiassny, an ichthyologist with the American Museum of Natural History in New York. It was a clear sign of decompression sickness, an often-fatal ailment that results when animals are rapidly depressurized. In humans, the buildup of nitrogen bubbles in the blood during a rapid ascent from deep water is called the bends.

“I thought, could this thing really be dying of the bends?” she said. “And if that was the case, how deep is the water here?”

The Congo River runs for 2,500 lazy miles through Africa’s equatorial basin, coiling like a snake through the jungles of the Democratic Republic of Congo. Then, 186 miles from the ocean, it drops into a dangerous series of gorges, sinking 12 feet every mile. The swirling waters of the Kongo Central province, bordering the Republic of Congo and Angola, contain 30 percent of the river’s fish diversity and have long attracted ichthyologists.