In the US, under the Federal Aviation Regulations (FAR) it’s usually illegal to fly under or even particularly near a “person, vessel, vehicle, or structure.” FAR 91.119 specifies the Minimum Safe Altitudes above and near man made objects.However, under certain conditions, it is legal to fly under a bridge here in the US.
Under that proviso I’ve flown under a particular bridge in California a number of times.
As a “seaplane” pilot it is legal to fly under a bridge if the act of flying under the bridge is required as part of a safe takeoff.
Cool, thanks for the response.