Megacities sinking

Subsiding land is a bigger immediate problem for the world's coastal cities than sea level rise, say scientists.
In some parts of the globe, the ground is going down 10 times faster than the water is rising, with the causes very often being driven by human activity.
Decades of ground water extraction saw Tokyo descend two metres before the practice was stopped.
Speaking at the European Geosciences Union General Assembly, researchers said other cities must following suit.
Gilles Erkens from the Deltares Research Institute, in Utrecht, in the Netherlands, said parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal urban settlements would sink below sea level unless action was taken.
His group's assessment of those cities found them to be in various stages of dealing with their problems, but also identified best practice that could be shared.
"Land subsidence and sea level rise are both happening, and they are both contributing to the same problem - larger and longer floods, and bigger inundation depth of floods," Dr Erkens told BBC News.
"The most rigorous solution and the best one is to stop pumping groundwater for drinking water, but then of course you need a new source of drinking water for these cities. But Tokyo did that and subsidence more or less stopped, and in Venice, too, they have done that."
The famous City of Water in north-east Italy experienced major subsidence in the last century due to the constant extraction of water from below ground.