About six million years ago the Mediterranean was cut off from the Atlantic and nearly dried up, leaving a desert of salt kilometers below sea level. Here is how a drill ship found it, the flood that refilled the sea, and the architect who wanted to lower it on purpose.

In the autumn of 1917, British schoolchildren were paid to gather horse chestnuts by the ton, and nobody would tell them why. The conkers were bound for a secret factory that fermented them into acetone, the solvent behind the cordite that fired British guns. Here is how the scheme worked, why it fell short, and how children in America and Germany were put to similar work.

For about a century, cities from London to Paris to New York moved letters and telegrams through miles of pipes under their streets, pushed by compressed air. Here is how pneumatic tube mail worked, what it carried, including one very unhappy cat, and why trucks won in the end.

In 1938 Lise Meitner, a refugee in Sweden, worked out with her nephew Otto Frisch why uranium had split in two. Hahn alone got the Nobel Prize. Nominated 49 times, she never won, but her name is now on the periodic table.

A Swedish chemist’s dazzling green ended up on Victorian walls, gowns, sweets and book covers. It was made with arsenic. Here is how the craze spread, who paid for it, and why libraries are still checking their shelves.

In 1936 Danish seismologist Inge Lehmann used faint waves from a New Zealand earthquake to find a core within Earth’s core. Nearly a century later, that iron heart is still surprising scientists.

Harry Houdini spent his last years exposing fraudulent spirit mediums, from Conan Doyle’s séances and the Margery case to Congress. A century later, the science of why we get fooled.

A century after her thesis, a London blue plaque honors the astronomer who found that stars are mostly hydrogen and helium, and the famous astronomer who told her it was impossible.

Henrietta Swan Leavitt found the Cepheid law that first measured distances to other galaxies. A century later, astronomers rechecking her handwritten data say it holds up remarkably well.

In 1961 a converted barge off Mexico drilled into the deep ocean floor for the first time, the opening move in Project Mohole, a Cold War plan to drill through Earth’s crust to the mantle. Mohole collapsed in politics and cost overruns, but the ocean drilling it spawned proved plate tectonics, found a dried-up Mediterranean and, 65 years on, is still chasing the Moho.

Minutes before the first Moon landing, the computer flying Apollo 11’s lunar module began flashing a 1202 alarm. The story of that machine, its programs woven by hand into core rope, and the MIT team led by Margaret Hamilton whose software knew what to drop and kept flying.

In June 1783 a crack opened in the Icelandic highlands and poured out lava and poison gas for eight months. The famine that followed killed a sixth of Iceland, a stinking dry fog spread over Europe, and Benjamin Franklin guessed what it meant.

For 130 years, the kilogram was a golf-ball-sized cylinder locked in a vault near Paris. Then it seemed to start losing weight, and scientists spent four decades learning to weigh with electricity and count atoms to replace it.

In a Montreal operating room, a surgeon touched a wire to the brains of awake patients and asked what they noticed. Their answers gave neuroscience its most famous map, and one of its most stubborn myths.