In which the writer discovers some little-known information in very small type at the bottom of a page, and embarks on the research project of a lifetime.

The most interesting journey of exploration in my writing life began with a footnote.

A book I was reading quoted something by Mark Twain. At the bottom of the page, a note said that Twain had written the passage when he was active at the turn of the century in the worldwide movement against atrocities in the Congo -- events that had taken 5 to 8 million lives.

I was taken aback. I knew that the European conquest of Africa had been a bloody business, but so many lives in just one territory? And why hadn't I heard about this before? I'd been to Africa half-a-dozen times as a journalist, once even to the Congo itself. Soon after, I pulled off my shelf a copy of Hannah Arendt's The Origins of Totalitarianism, where there was a passing reference to the conquest of the Congo by King Leopold II of Belgium. She cited an even higher death toll, of at least 12 million, "the blackest pages in the history of Africa." But she, too, mentioned this astounding number only in a footnote.

When I finally began digging into the subject, I found that the best demographic estimate today is that, between 1880 and 1920, the population of the Congo was slashed by about 10 million people. At the core of this catastrophe was, of all things, the invention of the inflatable bicycle tire, in 1887. This soon set off a worldwide rubber boom, just after King Leopold II had succeeded in seizing this vast territory, whose rain forest was immensely rich in rubber growing wild. Leopold ordered his private army into village after village. Soldiers would hold the women hostage -- in order to force the men to go into the forest, for days and eventually weeks out of every month, for the painstaking work of gathering wild rubber. Many hostages were raped and starved; many rubber workers were worked to death. More people died when soldiers shot down Africans who took part in numerous rebellions. Famines raged, for with women held hostage and men working as forced laborers, there were few people to hunt, fish or grow food. And the most deaths of all came because when people are near starvation, they die from diseases they might otherwise have survived.

I not only had a shrewd, sly, greedy king as the villain of this story, it turned out I also had an extraordinary collection of heroes: Black and white, African, European and American, they had tried to alert the world to Leopold's horrendous slave labor system. I began feeling as if I were a novelist but that someone else had invented my cast of characters and had handed them all to me on a platter. Many scholars and biographers had bitten off pieces of the Congo story, but, amazingly, no one had written a general-audience book in English about the entire drama. I'm still baffled by this.

Some of the most fascinating material was hidden in obscure places. A highly scholarly edition of an unfinished manuscript, E.D. Morel's History of the Congo Reform Movement, edited with numerous appendices by two professors and probably read in its entirety only by a few dozen people, contained a riveting autobiographical description that brought tears to my eyes. Morel, a junior official of a British shipping line, is sent to the Belgian port of Antwerp to supervise the loading and unloading of his company's ships as they come in from the Congo. They arrive packed to the hatch-covers with valuable cargoes of ivory and rubber. But, Morel notices, they return to the Congo carrying almost entirely soldiers, firearms and ammunition. Horrified, he realizes that there is no trade going on here. Nothing is being exchanged for the rubber and ivory. It can have only one possible source: slave labor. Blazing with indignation, Morel then quits his job, picks up his pen, rejects an attempted bribe from the king, and devotes the next dozen years of his life to putting Leopold's slave labor system on the world's front pages. Was there any doubt about the scene my book should begin with? This is what I mean by the story being handed to me on a platter. I later made a pilgrimage to that dock in Antwerp.

I had no idea how much sheer pleasure it would be to do the research. There's something especially enjoyable when the time you're working with is around a hundred years ago or more; with few or no telephones, people wrote everything down. Every visitor to the Congo, it seemed, spent each evening on the river bank writing letters home. And nobody seemed to have any idea how incredibly revealing their diaries would be when read a century later. When an officer named Georges Bricusse ordered and watched the hanging of an African who had stolen a rifle, he confided jubilantly to his journal, "It didn't make the least impression on me this time!! And to think that the first time I saw the whip administered, I was pale with fright. Africa has some use after all. I could now walk into fire as if to a wedding."

This and much more was all there waiting for me, some of it in archives or on microfilm, much of it already published in ancient dusty volumes. For one book I took out of the library, the previous borrower had been in 1937; several others from early in the century had pages still uncut. To trace the links between King Leopold's Washington lobbyist and U.S. president Chester A. Arthur, whose government was the first to recognize the king's avaricious claim to the entire Congo as his privately owned colony, I found myself reading newspapers from 1883. A group of reporters had been on the train that took Arthur on a vacation visit to the lobbyist's lavish Florida orange plantation. More delicious details passed to me on the platter -- from what kind of fish the president caught to the image of the secretary of the Navy climbing a tree to pick a particularly tasty-looking orange.

My most enjoyable detective work had to do with Joseph Conrad. For it is King Leopold's Congo that Conrad is writing about in Heart of Darkness. At the novel's center is the phantasmagoric figure of Kurtz, the fenceposts outside his house topped with severed African heads.

But was Conrad's famous villain a figure only of fantasy?

As I spent several years reading everything I could find in the way of memoirs, diaries, letters and studies having to do with the turn-of-the-century Congo, I began to notice one account after another of white men who boasted about collecting severed African heads. Four such men, in fact: one of whom even packed an African's head in a box of salt and sent it to be stuffed and mounted by his London taxidermist.

Conrad's numerous biographers had paid virtually no attention to these four men. As with all literary biographers, their sources are mainly the writer's diaries, letters to and from Conrad, and accounts by people who knew him. In this respect, Conrad's six months as a Congo steamboat officer is mostly a blank spot: He was unpublished and unknown, so no one paid him much attention. He also became deathly ill, and very few letters from him during that period survive.

I tried to find out everything I could about these four white head collectors -- and to see whether Conrad might have met or heard of any of them. In three cases, I struck pay dirt. Guillaume Van Kerckhoven was a notoriously aggressive officer in King Leopold's private army who once boasted to a fellow steamboat passenger that he paid his African conscript soldiers "5 brass rods . . . per human head they brought him during the course of any military operations he conducted. He said it was to stimulate their prowess in the face of the enemy." The fellow passenger Van Kerckhoven was chatting with was a young Irishman named Roger Casement, himself later to become a major crusader for justice in the Congo. Three years after his steamboat trip with Van Kerckhoven, he and Conrad shared a house for some days and the two men became close friends. Of Casement, the novelist once wrote: "He could tell you things! Things I have tried to forget . . . " Was one of those "things I have tried to forget" the story of Van Kerckhoven and the human heads?

Another head collector, Leon Fievez, had once boasted of "a hundred heads cut off" at his orders. Could Conrad have met him? The Belgians, to celebrate their glorious good works in the Congo, have published several biographical encyclopedias of colonial officials; one of them runs to eight volumes. I consulted these: During Conrad's visit to the Congo, it turned out, Fievez was commander of a heavily fortified post where Conrad's steamboat almost certainly stopped for fuel, going both up and down river.

The most interesting of these men was another officer in Leopold's army, a young Belgian named Leon Rom. Rom's head collecting has briefly caught the eye of a handful of Conrad critics, because a shocked British explorer once wrote about the 22 severed African heads that Rom had as a border around his flower bed. But none of the critics seem to have tried to find out anything else about Rom -- or whether Conrad could have met him.

I wrote to a colonial museum in Belgium to ask if they had any of Rom's letters. Weeks went by. I had forgotten about the letter, and forgotten that my fax number is in my letterhead. Then I woke up one morning and was thrilled to find all the pages of Rom's personal notebook in my fax machine. It showed, among other things, that he had been commander of an extremely small outpost that we know Conrad passed through on Aug. 1, 1890. It would have been remarkable if the two white men had not talked.

Furthermore, the museum wrote, it had five portraits and landscapes Rom had painted. My excitement mounted, for the Kurtz of Heart of Darkness was also a painter. Then, in reading a turn-of-the-century Belgian colonial-heroic writer, I found a reference to Rom as an avid collector of Congo butterflies; he was even a member of the Entomological Society of Belgium. Conrad's Kurtz also had scientific pretensions. I thought I had my man.

Some months later, one more idea occurred to me. Kurtz was also a poet and writer. Did Leon Rom also write anything? It seemed unlikely, for I'd been poring through endless numbers of old books, magazines and newspapers about Leopold's Congo without ever noticing Rom's byline. But I had never actually looked him up in the inter-library electronic catalog, which links the holdings of more than 20,000 libraries in the United States and abroad. I typed in Rom's name, waited, and there he was on the screen! Author of Le Negre du Congo, 1899.

When I thought to do this search, I was sitting in the superb Africa library at Northwestern University. It was 10 p.m., the library closed at midnight, and early the next morning I had to be on a plane back to my home in California. By a stroke of good luck, it turned out that this was the one library in North America that had a copy of Rom's book. I let out a suppressed yelp, raced for the shelf, found the book -- had anyone else borrowed it since 1899? I was so excited I forgot to look -- and photocopied the entire thing before the midnight deadline.

My seatmate on the plane home the next morning must have been puzzled by the avidity with which I pored over these pages. Conrad probably never read Le Negre du Congo; it appeared in Belgium at almost exactly the same time as Heart of Darkness was first published in a British magazine. In the novel, Kurtz writes a lengthy report to the Society for the Suppression of Savage Customs. Le Negre du Congo is Rom's own account -- crude, racist, arrogant, simple-minded -- of what he thinks of as savage customs. Leon Rom had obviously inspired Conrad in his creation of Kurtz, but then the real-life Rom had gone on to resemble the fictional Kurtz even more closely. Life had imitated art. Conrad would have been pleased.

At a reading a few months ago, someone asked me if I used a research assistant in writing King Leopold's Ghost. Are you kidding? Give away all this fun to someone else? Never!