Each year millions of visitors walk through the cobbled streets of Prague's Old Town - without realising, most likely, that many of the stones below their feet have been looted from what was meant to be sacred ground. The BBC's Rob Cameron only recently learned their secret.
We stood, blocking the pedestrian traffic, on one of the busiest streets in the Czech capital. A steady stream of people pushed by us muttering as they clutched bags of Christmas shopping and souvenirs and we peered at the ground.
In the distance, at the bottom of Wenceslas Square, crowds congregated around street performers and kiosks selling sausages and beer.
"There," said Leo Pavlat, the owlish, bearded director of the Prague Jewish Museum, pointing at a thin strip of dark, square cobblestones at our feet. "There! You see? All along there." He looked up, his eyes following the strip as it ran along the short pedestrianised street.
He delved into a plastic bag and brought out two cobblestones. They were almost identical to those embedded in the ground below us. But these ones you could turn over in your fingers, revealing a single smooth side of polished granite that would otherwise have been hidden face down.
One bore fragments of a date, 1895. The other featured three letters of the Hebrew alphabet - he, vav, bet, the gold paint which lined the chiselled inscriptions glinting in the winter sun.
"What does it mean?" I asked. "Is it part of a name?" Leo frowned. "No idea. It's not enough to tell. Possibly it's part of a eulogy."
And across the country, on the edges of villages and towns, some 600 Jewish cemeteries lay untended and forgotten. The Communist authorities - and, it seems, officials within the Jewish community too - saw them as repositories of valuable building material that would otherwise go to waste.
Leo Pavlat couldn't remember where his stones had come from, but directed me to an article he'd written several years before. His cobbles, it seems, were cut from tombstones taken from a Jewish cemetery established in 1864 in the town of Udlice in North Bohemia.
There'd been a Jewish community there since the 17th Century, with a synagogue, yeshiva (a religious school) and two cemeteries. By 1930, the Jewish population of Udlice had fallen to 13. By the 1980s, when its cemetery was looted, it was - presumably - zero.
After a few minutes' walk, we reached the end of the granite line, at the bottom of Wenceslas Square. Tourists and locals jostled past us.