Kitum Cave is one of five "elephant caves" on Mount Elgon, the eroded volcano straddling the Kenya-Uganda border. It is not a lava tube, as people sometimes assume, but a cave formed in pyroclastic rock - the compacted debris of ancient eruptions - and it runs about two hundred meters into the mountainside in Kenya's Mount Elgon National Park. The salt-rich walls draw more than elephants. Bushbuck, buffalo, and hyenas come to lick the rock and the salt the elephants leave behind, turning the cave into a kind of communal mineral lick. Deeper inside, the air thickens with the smell of bats, and the floor is layered with their guano. It is a strange, dark, living place - and a dangerous one. Somewhere in the cave is a deep crevasse, and young elephants have fallen in and died.

In the 1980s, the cave became the center of a medical mystery that would unsettle the world. Two people who visited Kitum fell gravely ill afterward with Marburg virus disease, a rare and ferocious hemorrhagic fever related to Ebola. In 1980, a French man died after visiting the cave. In 1987, a Danish boy living in Kenya also visited, fell ill, and died. They deserve to be remembered as people, not just case files - a man and a child who walked into a beautiful, ordinary-seeming cave and did not survive the encounter. Each left a scientific legacy in the most somber way: two closely related viruses isolated from these infections were named for the cases - one for Dr. Shem Musoke, who survived after being infected by the French patient, and one, called Ravn, for the Danish boy.

Two deaths, years apart, in the same remote cave - it was the kind of pattern that demanded an answer. The United States Army Medical Research Institute of Infectious Diseases mounted an expedition to Kitum to find whatever was carrying the virus, sampling animals and insects, including the cave's fruit bats. They found nothing. The vector stayed hidden, and the cave kept its secret. It was this search that the writer Richard Preston dramatized in his 1994 bestseller The Hot Zone, which fixed Kitum Cave in the public imagination as a place where something deadly waited in the dark. The book made the cave famous. It did not solve the riddle.

The resolution came two decades later, and from elsewhere. In 2007, after two miners contracted Marburg in active mines in Gabon and Uganda, researchers found clear evidence of the virus living in cave-dwelling Egyptian fruit bats. The Ugandan mines harbored colonies of the very same bat species that roosts in Kitum Cave - strong evidence that the bats, and the fine dust of their dried guano, had been the long-sought reservoir all along. Tellingly, neither miner had been bitten, which suggested the virus could spread simply by breathing the powdered guano. The mystery that had haunted Kitum since the 1980s finally had a likely answer. The elephants still come at night to mine their salt, indifferent to the science. The cave remains what it has always been: a place of genuine wonder, and of caution earned the hard way.