Bermuda Triangle: May I use the progress card "Road Building" to build bridges?
The metropolis is still a city – although a very large one. Maybe the barbarians stay clear of it, but lava doesn't. The metropolis is thus downgraded to a settlement. Afterwards, the player may immediately declare another city metropolis, or he gets the metropolis back when he builds the next city (that is, if another player hasn't built the metropolis in the meantime).