For more than a hundred years, farmers, colonial surveyors and trained field biologists have described the same small upright ape in the forests of Sumatra. There are footprint casts, hair samples and a fifteen-year camera-trap project. There is still not one photograph, and not one bone.

Between 1764 and 1767, something hunted people in the highlands of south-central France. It killed around a hundred of them, most of them women and children. A king sent his own hunters, two animals were shot and paraded as the culprit, and no one ever proved what it was.

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.

Again and again, hundreds of healthy whales swim as one onto the same fatal beaches and refuse to leave, even after rescuers drag them back to sea. Decades of science have offered sonar, solar storms, sick leaders and treacherous sand. Not one of them fully explains why the pods keep dying together.

For a few nights in April 2005, more than a thousand toads in a quiet Hamburg pond swelled to bursting and detonated, flinging their entrails a full metre across the water. A Berlin vet named a culprit nobody had imagined. The official file still reads: no satisfactory explanation.

For over a century, shapeless masses of flesh with no bones and no eyes have washed ashore and been mistaken for giant octopuses and hairy sea beasts. DNA finally read the label.

A spiny alien that drained goats of their blood terrified Puerto Rico, then all of Latin America. The trail led back to a horror film and a mangy coyote.

A newspaper sensation, a toy submarine, a fleet of sonar boats and a survey of the water's own DNA. Strip away the hoaxes and what is really left in Loch Ness?

For centuries the Himalayas held out a giant with a human tread. Then geneticists asked the hair, the bones and the monastery relics to speak for themselves.

In 1938 a curator picked a steel-blue fish out of a South African trawler's catch. It belonged to a lineage the world had buried with the dinosaurs, and it turned out not to be alone.

For twenty minutes in the clear light of an August afternoon in 1848, seven officers of a British frigate watched a sixty-foot serpent hold a steady course past their ship. Their captain reported it to the Admiralty in writing and never once backed down.

Sailors swore a tentacled monster could drag ships under. Naturalists laughed. Then the carcasses started washing ashore, and cameras finally caught it alive in the deep.