https://unsolvedcatalog.org A catalog of open questions. en https://unsolvedcatalog.org/053-why-are-the-moons-two-faces-so-different/ https://unsolvedcatalog.org/053-why-are-the-moons-two-faces-so-different/ Sun, 16 Aug 2026 12:00:00 GMT The side of the Moon we see is dark lava plains; the side we never see is pale, cratered, and thicker-skinned. Sixty years of spacecraft have not settled why. https://unsolvedcatalog.org/052-how-does-lightning-get-started/ https://unsolvedcatalog.org/052-how-does-lightning-get-started/ Sun, 16 Aug 2026 12:00:00 GMT The electric fields measured inside thunderclouds are roughly ten times too weak to spark. Lightning strikes anyway, about a hundred times a second, worldwide. https://unsolvedcatalog.org/051-where-do-european-eels-spawn/ https://unsolvedcatalog.org/051-where-do-european-eels-spawn/ Sun, 16 Aug 2026 12:00:00 GMT No one has ever seen a European eel egg, or a European eel spawning, in the wild. Aristotle guessed. We are still guessing better. https://unsolvedcatalog.org/050-how-does-gravity-fit-with-quantum-mechanics/ https://unsolvedcatalog.org/050-how-does-gravity-fit-with-quantum-mechanics/ Sun, 16 Aug 2026 12:00:00 GMT Physics runs on two magnificent theories that contradict each other about what reality is. Every attempt to marry them remains unconfirmed by a single experiment. https://unsolvedcatalog.org/049-does-hot-water-freeze-faster/ https://unsolvedcatalog.org/049-does-hot-water-freeze-faster/ Sun, 16 Aug 2026 12:00:00 GMT Aristotle said yes. A Tanzanian schoolboy insisted yes. Careful labs say maybe not. This entry is unusual: the open question is whether the effect exists at all. https://unsolvedcatalog.org/048-why-do-we-have-fingerprints/ https://unsolvedcatalog.org/048-why-do-we-have-fingerprints/ Sun, 16 Aug 2026 12:00:00 GMT The ridges on your fingertips are famous for identifying you. That is a use humans invented. What they are actually for is still being tested. https://unsolvedcatalog.org/047-what-killed-the-ice-age-giants/ https://unsolvedcatalog.org/047-what-killed-the-ice-age-giants/ Sun, 16 Aug 2026 12:00:00 GMT Mammoths, giant sloths, sabertooth cats: the large animals of every continent except Africa vanished in a geological instant. The suspects are climate and us. https://unsolvedcatalog.org/046-what-does-whale-song-mean/ https://unsolvedcatalog.org/046-what-does-whale-song-mean/ Sun, 16 Aug 2026 12:00:00 GMT Humpback males sing structured, evolving songs that sweep across oceans like hit records. Half a century of listening has not settled what the songs accomplish. https://unsolvedcatalog.org/045-what-is-mecfs/ https://unsolvedcatalog.org/045-what-is-mecfs/ Sun, 16 Aug 2026 12:00:00 GMT An illness that follows infection, worsens with exertion, and disables millions has no biomarker, no approved treatment, and no accepted mechanism. The patients were right; the science is late. https://unsolvedcatalog.org/044-where-does-the-protons-spin-come-from/ https://unsolvedcatalog.org/044-where-does-the-protons-spin-come-from/ Sun, 16 Aug 2026 12:00:00 GMT The proton spins. Its three quarks, added up, provide barely a third of the total. Most of the spin of ordinary matter is bookkept somewhere still unmeasured. https://unsolvedcatalog.org/043-why-do-we-blush/ https://unsolvedcatalog.org/043-why-do-we-blush/ Sun, 16 Aug 2026 12:00:00 GMT An involuntary broadcast of exactly the feeling you most want to hide, wired into one species only. Darwin called it the most human expression, and he could not explain it. https://unsolvedcatalog.org/042-where-do-the-highest-energy-particles-come-from/ https://unsolvedcatalog.org/042-where-do-the-highest-energy-particles-come-from/ Sun, 16 Aug 2026 12:00:00 GMT Single atomic nuclei arrive from space carrying the energy of a fast-pitched baseball. Nothing we know of should be able to throw them. https://unsolvedcatalog.org/041-how-do-rogue-waves-form/ https://unsolvedcatalog.org/041-how-do-rogue-waves-form/ Sun, 16 Aug 2026 12:00:00 GMT Sailors' tales of walls of water were dismissed for centuries, until a drilling platform measured one. The ocean makes monsters; the recipe is disputed. https://unsolvedcatalog.org/040-what-causes-deja-vu/ https://unsolvedcatalog.org/040-what-causes-deja-vu/ Sun, 16 Aug 2026 12:00:00 GMT For a few seconds, the present moment feels like a memory, and you know it isn't. The glitch that watches itself has no confirmed mechanism. https://unsolvedcatalog.org/039-why-cant-humans-regrow-limbs/ https://unsolvedcatalog.org/039-why-cant-humans-regrow-limbs/ Sun, 16 Aug 2026 12:00:00 GMT An axolotl rebuilds a severed leg, perfectly, bones and nerves and all. Mammals seal the wound and call it done. Both use nearly the same genes. https://unsolvedcatalog.org/038-what-causes-migraines/ https://unsolvedcatalog.org/038-what-causes-migraines/ Sun, 16 Aug 2026 12:00:00 GMT A billion people carry a brain that periodically attacks them, complete with light shows and a warning phase. The full circuit has never been traced. https://unsolvedcatalog.org/037-how-fast-is-the-universe-expanding/ https://unsolvedcatalog.org/037-how-fast-is-the-universe-expanding/ Sun, 16 Aug 2026 12:00:00 GMT Two impeccable measurements of the universe's expansion rate disagree, and the disagreement has survived every attempt to blame the instruments. https://unsolvedcatalog.org/036-what-makes-sonoluminescence-glow/ https://unsolvedcatalog.org/036-what-makes-sonoluminescence-glow/ Sun, 16 Aug 2026 12:00:00 GMT Squeeze a bubble in water with sound waves and it emits flashes of light, from a spot briefly hotter than the surface of the Sun. The lamp works; the manual is unfinished. https://unsolvedcatalog.org/035-how-many-species-are-there/ https://unsolvedcatalog.org/035-how-many-species-are-there/ Sun, 16 Aug 2026 12:00:00 GMT Around 2.2 million species have names. The true total is unknown to within an order of magnitude, and possibly several. https://unsolvedcatalog.org/034-what-happens-to-information-in-a-black-hole/ https://unsolvedcatalog.org/034-what-happens-to-information-in-a-black-hole/ Sun, 16 Aug 2026 12:00:00 GMT Quantum mechanics forbids information from being destroyed. Black holes looked like they destroy it. The verdict has shifted; the mechanism is still missing. https://unsolvedcatalog.org/033-what-is-the-glass-transition/ https://unsolvedcatalog.org/033-what-is-the-glass-transition/ Sun, 16 Aug 2026 12:00:00 GMT Cool a liquid past crystallization without letting it crystallize and it quietly becomes solid without ever changing structure. Physics cannot say what kind of event that is. https://unsolvedcatalog.org/032-why-is-yawning-contagious/ https://unsolvedcatalog.org/032-why-is-yawning-contagious/ Sun, 16 Aug 2026 12:00:00 GMT You may yawn while reading this entry. Nobody can tell you why, on either count: what yawns are for, or why they spread. https://unsolvedcatalog.org/031-why-do-humans-cry-emotional-tears/ https://unsolvedcatalog.org/031-why-do-humans-cry-emotional-tears/ Sun, 16 Aug 2026 12:00:00 GMT Every animal with eyes lubricates them. Only humans weep at funerals, weddings, and films. The function of the most human fluid is unestablished. https://unsolvedcatalog.org/030-why-do-cats-purr/ https://unsolvedcatalog.org/030-why-do-cats-purr/ Sun, 16 Aug 2026 12:00:00 GMT The most familiar sound in domestic life, produced by a mechanism only recently glimpsed, for reasons that remain genuinely unclear. https://unsolvedcatalog.org/029-how-do-high-temperature-superconductors-work/ https://unsolvedcatalog.org/029-how-do-high-temperature-superconductors-work/ Sun, 16 Aug 2026 12:00:00 GMT Certain ceramics conduct electricity with zero loss at temperatures far too warm for the standard theory to allow. Four decades on, the mechanism is still contested. https://unsolvedcatalog.org/028-why-does-time-flow-one-way/ https://unsolvedcatalog.org/028-why-does-time-flow-one-way/ Sun, 16 Aug 2026 12:00:00 GMT The laws of physics run fine in reverse. Eggs still refuse to unscramble. The one-wayness of time is real, measurable, and not yet explained at the bottom. https://unsolvedcatalog.org/027-what-are-fast-radio-bursts/ https://unsolvedcatalog.org/027-what-are-fast-radio-bursts/ Sun, 16 Aug 2026 12:00:00 GMT Millisecond flashes of radio from other galaxies, each briefly outshining its host. Thousands are catalogued. The engine is still not identified. https://unsolvedcatalog.org/026-what-triggered-the-cambrian-explosion/ https://unsolvedcatalog.org/026-what-triggered-the-cambrian-explosion/ Sun, 16 Aug 2026 12:00:00 GMT Life spent three billion years as slime, then produced most animal body plans in a geological moment. Darwin called it a valid objection to his theory. The trigger is still not settled. https://unsolvedcatalog.org/025-how-do-memories-outlive-their-molecules/ https://unsolvedcatalog.org/025-how-do-memories-outlive-their-molecules/ Sun, 16 Aug 2026 12:00:00 GMT The proteins storing your oldest memory were recycled thousands of times since it formed. Something persists. What, exactly, is the question. https://unsolvedcatalog.org/024-why-do-zebras-have-stripes/ https://unsolvedcatalog.org/024-why-do-zebras-have-stripes/ Sun, 16 Aug 2026 12:00:00 GMT Darwin and Wallace argued about it. A century and a half later the leading answer involves flies, and the mystery has moved rather than died. https://unsolvedcatalog.org/023-does-every-number-fall-to-one/ https://unsolvedcatalog.org/023-does-every-number-fall-to-one/ Sun, 16 Aug 2026 12:00:00 GMT Halve it if even, triple it and add one if odd, repeat. A child can play the game. Nobody alive can prove it always ends. https://unsolvedcatalog.org/022-why-is-there-more-matter-than-antimatter/ https://unsolvedcatalog.org/022-why-is-there-more-matter-than-antimatter/ Sun, 16 Aug 2026 12:00:00 GMT The Big Bang should have made matter and antimatter in equal amounts, which then annihilate to light. Something cheated, one part in a billion. You are made of the discrepancy. https://unsolvedcatalog.org/021-where-did-earths-water-come-from/ https://unsolvedcatalog.org/021-where-did-earths-water-come-from/ Sun, 16 Aug 2026 12:00:00 GMT The planet formed close to a young star, hot and dry by every rule we know. Yet here are the oceans. The delivery route is still being argued. https://unsolvedcatalog.org/020-how-does-the-placebo-effect-work/ https://unsolvedcatalog.org/020-how-does-the-placebo-effect-work/ Sun, 16 Aug 2026 12:00:00 GMT A sugar pill, honestly labeled as nothing, can still relieve pain. The body runs a pharmacy that expectation can unlock, and we hold only a partial inventory. https://unsolvedcatalog.org/019-what-is-consciousness-physically/ https://unsolvedcatalog.org/019-what-is-consciousness-physically/ Sun, 16 Aug 2026 12:00:00 GMT Three pounds of matter produces an inner life. Every fact in this catalog was experienced by someone. How that happens is unknown at the deepest level we can ask. https://unsolvedcatalog.org/018-why-do-we-hiccup/ https://unsolvedcatalog.org/018-why-do-we-hiccup/ Sun, 16 Aug 2026 12:00:00 GMT A reflex with its own dedicated neural circuitry, present before birth, apparently good for nothing. It has a job or it is a souvenir. Nobody knows which. https://unsolvedcatalog.org/017-how-did-language-begin/ https://unsolvedcatalog.org/017-how-did-language-begin/ Sun, 16 Aug 2026 12:00:00 GMT The defining human ability left no fossils. The question was once so hopeless that the field formally banned it. https://unsolvedcatalog.org/016-where-is-everybody/ https://unsolvedcatalog.org/016-where-is-everybody/ Sun, 16 Aug 2026 12:00:00 GMT Billions of old, sunlike stars. Planets around most of them. Six decades of listening. One universe, apparently silent. https://unsolvedcatalog.org/015-how-does-turbulence-work/ https://unsolvedcatalog.org/015-how-does-turbulence-work/ Sun, 16 Aug 2026 12:00:00 GMT We have had the exact equations for chaotic fluid flow since the 1840s. We still cannot solve them, and we cannot even prove they behave. https://unsolvedcatalog.org/014-what-causes-alzheimers-disease/ https://unsolvedcatalog.org/014-what-causes-alzheimers-disease/ Sun, 16 Aug 2026 12:00:00 GMT We can see the plaques, name the proteins, and now even slow the decline a little. The actual cause is still an open verdict. https://unsolvedcatalog.org/013-why-is-the-suns-corona-so-hot/ https://unsolvedcatalog.org/013-why-is-the-suns-corona-so-hot/ Sun, 16 Aug 2026 12:00:00 GMT Step back from a fire and the air gets cooler. Step off the Sun's surface into its atmosphere and the temperature rises three hundredfold. https://unsolvedcatalog.org/012-is-the-riemann-hypothesis-true/ https://unsolvedcatalog.org/012-is-the-riemann-hypothesis-true/ Sun, 16 Aug 2026 12:00:00 GMT One sentence from 1859 would, if proved, bring order to the prime numbers. It has been checked trillions of times and proved zero times. https://unsolvedcatalog.org/011-what-is-ball-lightning/ https://unsolvedcatalog.org/011-what-is-ball-lightning/ Sun, 16 Aug 2026 12:00:00 GMT Thousands of people have watched a glowing sphere drift through a room during a storm. Physics still cannot say what they saw. https://unsolvedcatalog.org/010-why-do-we-age/ https://unsolvedcatalog.org/010-why-do-we-age/ Sun, 16 Aug 2026 12:00:00 GMT Nothing in physics says a body must wear out. Some animals barely do. Ours does, on schedule, and the reason is still argued about. https://unsolvedcatalog.org/009-what-is-dark-energy/ https://unsolvedcatalog.org/009-what-is-dark-energy/ Sun, 16 Aug 2026 12:00:00 GMT About two thirds of the universe is something that pushes space apart. The leading explanation misses by a factor with 120 zeros. https://unsolvedcatalog.org/008-why-do-we-dream/ https://unsolvedcatalog.org/008-why-do-we-dream/ Sun, 16 Aug 2026 12:00:00 GMT Every night you hallucinate vividly for about two hours, then forget most of it. No one can tell you what that is for, or whether it is for anything. https://unsolvedcatalog.org/007-how-do-birds-sense-magnetic-fields/ https://unsolvedcatalog.org/007-how-do-birds-sense-magnetic-fields/ Sun, 16 Aug 2026 12:00:00 GMT A robin can feel a force you cannot. The leading explanation requires quantum mechanics operating inside a warm, wet eye. https://unsolvedcatalog.org/006-does-p-equal-np/ https://unsolvedcatalog.org/006-does-p-equal-np/ Sun, 16 Aug 2026 12:00:00 GMT If checking an answer is easy, is finding one easy too? A million-dollar question about the nature of difficulty itself. https://unsolvedcatalog.org/005-why-is-ice-slippery/ https://unsolvedcatalog.org/005-why-is-ice-slippery/ Sun, 16 Aug 2026 12:00:00 GMT A question a child can ask, studied since Faraday, still argued in physics journals. The textbook answer you learned is mostly wrong. https://unsolvedcatalog.org/004-how-did-life-begin/ https://unsolvedcatalog.org/004-how-did-life-begin/ Sun, 16 Aug 2026 12:00:00 GMT Somewhere on the early Earth, chemistry became biology. Every laboratory attempt to repeat the trick has stalled at the same gap. https://unsolvedcatalog.org/003-why-does-anesthesia-work/ https://unsolvedcatalog.org/003-why-does-anesthesia-work/ Sun, 16 Aug 2026 12:00:00 GMT We have been switching human consciousness off on demand since 1846. We still cannot say what the switch is. https://unsolvedcatalog.org/002-what-is-dark-matter/ https://unsolvedcatalog.org/002-what-is-dark-matter/ Sun, 16 Aug 2026 12:00:00 GMT Five independent lines of evidence say most of the matter in the universe is invisible. Fifty years of searching have not caught a single particle of it. https://unsolvedcatalog.org/001-why-do-we-sleep/ https://unsolvedcatalog.org/001-why-do-we-sleep/ Sun, 16 Aug 2026 12:00:00 GMT Every animal with a nervous system does it. It is dangerous, costly, and non-negotiable, and nobody knows what it is buying.