In 1890 Conrad passed an interview in Belgium with representatives of the Société Anonyme pour le Commerce du Haut-Congo and was then hired to go to the Congo to work on the river’s longest navigable reach, the thousand-mile section between what would later become Kinshasa, the city at the top of the rapids that guard the mouth of the Congo River, and Kisangani, the port city built as far up river as you can travel by boat before cataracts once again make the river impassable. Privately, Conrad wrote that childhood curiosity was one of his main reasons for going to the Congo. The western half of central Africa was one of the last parts of the continent to be penetrated by white outsiders and the region around the headwaters of the Congo River was, in his words, ‘the blankest of blank spaces on the earth’s figured surface’. But what he found when he got there was that the opening up process had cast the region not into light but into darkness.

The six months he spent in the Congo Free State left him scarred, both physically and mentally. To reach the bottom of the navigable stretch in 1890, there was no option other than to trek through two hundred miles of disease-ridden, parched mountain savannah to circumnavigate the cataracts on the lower river. In the diary he kept on his five-week-long yomp, he described how his group wandered listlessly along a trail marked by the bodies of dead natives, white officials routinely beat natives with staves and his European companion, Prosper Harou, was reduced within days to a pale, febrile wreck vomiting bile in a hammock borne by native bearers. When he reached the settlement built where Kinshasa stands today he found a tiny stockade, primitive, disorganised and staffed by colonial agents of dubious morals.

The river journey was even more wretched for Conrad. The skipper of his boat fell sick and Conrad had to take over, nursing the tiny steamboat each of the thousand miles upstream until finally reaching his company’s outpost at the foot of the falls. The company agent there, Antoine Klein, was ill and Conrad tried to save him by taking him back downriver, but the agent died en route. By the time Conrad got back to the lower settlement, he too was so ill he barely made it out of the Congo alive at the end of 1890, less than six months after he arrived. For the rest of his life his health suffered from the after-effects.

More profound than the physical ailments was the scarring on his soul from what he had seen. The secretive efforts of King Leopold’s collaborators had successfully concealed the true nature of the Congo Free State from the outside world, making the reality of its brutal oppression and wanton pillage all the more shocking to an outsider like Conrad.

It would be eight years until he began writing Heart of Darkness and all the key details of his own Congo experience were included in the story told by Marlow. Marlow does not name the place as the Congo, or even the continent as Africa, but he describes how he was hired by European agents of a colonial company and sent to a river with a long, navigable reach through wild rainforest, that was only accessible by route march from the coast and which culminated in a remote ‘Inner Station’ home to the most profitable of all the company’s ivory-collecting agents. In Conrad’s early drafts the agent had the same name as the dying man Conrad tried to rescue in 1890, Mr Klein. The name in the novel was only later changed to Mr Kurtz.