Why is the city of Norwich so vulnerable to sinkholes?

A series of sinkholes have opened up in Norwich over the decades, most famously in 1988 when a bus was swallowed up.

Since then, holes have regularly appeared in streets across the historic city, which was built on chalk.

Experts have warned those foundations have made it vulnerable, and further sinkholes cannot be ruled out.

University of East Anglia environmental sciences expert Prof Julian Andrews said rainwater flows into cracks in the chalk.

"[It] gradually enlarges them until a hole is formed or the top of the chalk collapses," he said.