The workers were digging a new drain beside the basketball court when a shovel struck stone. Under half a metre of soil lay a row of smooth cobbles, fitted together like a buried path. By lunchtime, the principal had fenced off the corner of the playground and called the city museum.

Dr Chen arrived with measuring tape, brushes and a folder of old maps. She explained that archaeologists did not begin by hunting for valuable objects. They began by recording where everything was found. Sofia's class watched her mark a grid over the trench. Within it, the team uncovered a glass bottle stopper, a clay marble and a brass button stamped with an anchor.

An 1891 map supplied the missing clue. Before the school existed, Harbour Lane crossed the same ground on its way to a busy produce market. The button probably came from a dockworker's coat; the marble may have belonged to a child waiting beside a market stall. Ordinary objects, Dr Chen said, could reveal who had actually used a place.

The drain still had to be built, so the stones could not remain exposed. The class photographed and mapped every cobble before the trench was covered. A month later, a brass line appeared across the new pavement, tracing the exact route of Harbour Lane. Students now crossed the old street every morning without needing to dig it up again.