The Arc de Triomphe in Paris, which the president vows to top in Washington, offers a lesson in the complicated history of monuments.

For centuries, the bones of some six million people were buried in the catacombs beneath the city. Curators are trying to preserve and modernize the tunnels while maintaining the spooky ambience.

Molenbeek was branded a hotbed of radicalization after terrorists hit Brussels and Paris a decade ago, but new challenges show that reinvention doesn’t come easy.