A man knocked down a wall in his basement in 1963 and opened a hidden city cut 85 m into volcanic rock, big enough for 20,000 people. Sixty years later, archaeologists still cannot say who dug the first tunnels.

In 1994 a German archaeologist walked onto a low mound in southeastern Turkey that everyone else had written off as a medieval graveyard. Under his feet lay carved stone circles raised by hunter-gatherers eleven and a half thousand years ago, and nobody can agree what they were for.

A diver hunting hammerhead sharks off Japan's westernmost island found a wall of rock with right angles and steps. Forty years later, two credentialled geologists still cannot agree whether anyone built it.

Brickyard workers in Sichuan hit a pit full of giant bronze faces, elephant tusks and gold. Every object had been deliberately broken and burned first, and the people who made them never wrote a single word.

A crater in the Turkmen desert has been burning for around fifty years, and millions of people know exactly how it started. The problem is that the story does not appear in a single document, and the geologists who worked there tell it differently.

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.

A 250 hectare wood on the edge of Cluj-Napoca is routinely called the most haunted forest on Earth. Behind the label sit four photographic negatives from August 1968, a circular clearing where trees will not grow, and decades of measurements that were never published. Here is what can actually be documented, and what cannot.

A magazine writer drew a triangle on the Atlantic in 1964. It took one librarian and two insurance ledgers to erase it.

For over a century, glowing orbs have danced across a Texas desert basin. Physicists traced most of them to a highway. Most — not all.