What is the world's most vulnerable city?

From the Maldives to Mauritania, some cities are engaged in a constant battle for survival against nature's relentless forces. But which of these metropolises is closest to being overwhelmed by sea, sand or other natural threat?

Setting aside epic disaster-movie moments such as volcanoes, hurricanes and earthquakes, there are two key natural factors that can make a city vulnerable to gradual disintegration, or even total disappearance - water and sand.

Were climate change making the planet colder rather than hotter, we could add ice to the list - for nothing obliterates a city like a billion-tonne glacier grinding its way down a valley. The impact of a rare "ice tsunami" in 2013 on the Canadian municipality of Ochre Beach was just a taster: a wall of melting iceberg on Dauphin Lake was blown by winds on to the shore, splintering every house in its path.