On a Sunday night in 1987, a grinning rubber mask seized two Chicago television stations and spoke gibberish to a city. Federal investigators had every advantage and never came close. Nearly forty years on, not one person has been identified.

In August 1991 NOAA tapped into the US Navy's undersea listening network and found a signal nobody could name. It has repeated ever since, peaking every spring and autumn, from an empty patch of the South Pacific. It is fading, and no ship has ever gone to look.

They last a thousandth of a second, blaze brighter than whole galaxies, and arrive from billions of light-years away. For years astronomers were not even sure they were real. We now know fast radio bursts exist, and one of the hardest questions in modern astronomy is what on earth is making them.

A distant star kept losing up to a fifth of its light in ragged, unpredictable dips. No planet could do that. For a while, even serious scientists whispered about alien engineering.

For over four decades, a shortwave frequency has broadcast a monotonous buzz from Russia, day and night. Occasionally a voice cuts in, reads coded names and numbers — and vanishes. No one has ever officially explained why.

In 1977, a radio telescope in Ohio caught a powerful narrowband signal from deep space. It lasted 72 seconds, looked exactly like an interstellar broadcast should — and it has never been heard again.

In Taos, Bristol, Windsor and dozens of other places, a low, relentless drone torments a small minority of residents — and decades of scientific investigation have never found a single source.

For decades, silent balls of light have floated over a remote Norwegian valley — and for over forty years, scientists with cameras, radar and magnetometers have been trying to explain them. They still can't.

Every October, glowing orbs rise silently from the Mekong before enormous crowds. After decades of theories — from tracer rounds to swamp gas — no one has proven what they are.

In late 2016, US diplomats in Cuba began reporting a sound no one else could hear, followed by symptoms no one could explain. Nearly a decade later, the government's own experts still cannot agree on what happened.

For centuries, coastal communities from Bengal to Belgium to the Carolinas have heard artillery booms with no guns, no storms and no aircraft in sight. Modern seismometers have ruled out the ground — and deepened the puzzle.

In 1997, hydrophones picked up a sound in the South Pacific louder than any known animal. For years it fed dreams of sea monsters. The real answer came from Antarctica.