Picture a scorpion a full meter long, with claws 16 centimeters each, hunting in a shallow prehistoric sea. It's not a scene from a horror film, but science's latest discovery - Praearcturus gigas, the largest scorpion that ever existed, which walked the ground of present-day Britain about 415 million years ago.

The most interesting part is how it was "discovered." The fossils had actually lain for more than a century in a natural history museum in London, misclassified. Back in the 1870s, someone declared them isopods - a kind of crustacean. Only now, with computed tomography and a re-analysis of eight specimens from three sites, did scientists realize what they were actually dealing with.

The key evidence was found by Richard Howard, the museum's curator of fossil arthropods - a long, triangular breastbone with a groove down the middle, the same as in a related Canadian scorpion. The body was covered with coarse bumps, typical of scorpions, and the claws large enough to provoke unpleasant thoughts. "You wouldn't want to meet a creature like this in a dark alley," jokes paleobiologist Russell Bicknell.

According to the researchers, published on June 2 in the journal Palaeontology, the beast probably led a semi-aquatic life and hunted primitive jawless fish - since small prey simply wouldn't have been enough for so large a body. Howard explains the absence of some parts with a neat image: "If you find a dinosaur skeleton without a head, you won't assume it had no head."

There's something soothing about news like this amid today's headlines of wars and crises. A reminder that the planet carries stories far older than all our borders and conflicts - and that some of the greatest discoveries don't come from new digs, but from looking more closely at what we already have on the shelf.