For more than a hundred years, expeditions have pushed into the flooded forests of the Congo Basin after a large unknown animal. They came back with recordings, blurred film and hundreds of interviews. They never came back with the animal, and the dinosaur story attached to it may not have started in Africa at all.

A Hungarian count gave away his library in 1838. Inside it sat a small, fat book written in a script that matches no alphabet on earth. Nearly two centuries and a dozen rival decipherments later, not one sentence of it has been read in a way other scholars accept.

A hotel maid found the greatest electrical inventor of the age dead and penniless in a small Manhattan room. Within two days federal agents had hauled his papers away in trucks. What they took, what they returned, and what never reached Belgrade is still argued about today.

In the 1930s, bulldozers clearing jungle for banana plantations in southern Costa Rica began turning up giant stone balls, some taller than a person and weighing as much as a loaded truck. Workers dynamited many of them looking for gold inside. Nearly a century later, nobody can say for certain how they were carved so precisely, or what they were for.

At eleven on a Sunday morning in 1978, something detonated over a small iron-mining island in Newfoundland. Fuse boxes threw blue flame across rooms, televisions burst, a shed came off its foundation with the chickens inside, and two neat holes were punched into a field. American satellites built to catch nuclear tests registered the flash, and Los Alamos sent scientists. Nearly fifty years later the file is still open.

More than a hundred hollow bronze dodecahedra have been dug up across the northern Roman provinces, each pierced by holes of different sizes. Not one Roman text, inscription or image mentions them. In 2023 a field in Lincolnshire produced the finest example yet, and the mystery only deepened.

On a Victorian bridge outside Dumbarton, dogs have been going over the same few metres of parapet for seventy years, almost always on clear days, almost always long-nosed breeds. In 2005 two specialists went looking at the bridge through a dog's eyes and came back with an answer involving a smell no human can detect. The answer still has holes in it.

For more than a hundred years, travellers crossing outback Queensland have described a light that rises out of the dark, hovers above the horizon, and follows them for kilometres. In 1992 a neuroscientist built one to order, using nothing but a car and ten kilometres of empty road. Boulia was not entirely grateful.

For generations the villagers of Shiyan Beicun believed their ponds were bottomless. In 1992 four of them bought pumps to find out, and drained the water off the roof of a hand-carved hall the size of a church. Twenty-four of them have now been opened, and not one line of Chinese history mentions that they exist.

Across the highlands of northern Laos stand thousands of carved stone jars, some weighing fourteen tonnes. A French archaeologist called them tombs in the 1930s. Then the bombs fell, and it took eighty years to get back to the question.

In July 1970 a US test missile carrying radioactive cobalt came down four hundred miles inside Mexico, and the American army built a railhead, a road and a camp to take it home. What stayed behind in the Chihuahuan Desert was a story about radios that will not work. Here is the documented record, and exactly where it runs out.

Ninety-two artificial islands sit in a lagoon off Pohnpei, walled with basalt logs that weigh up to fifty tonnes each. Dating puts the first courses at around 1180 CE. Nobody has yet explained how the stone crossed the island and floated onto the reef.