The added weight of structures is causing the land to subside.
Miami is apparently special, being one of the few spots in the world where buildings are making the entire city sink. But it only started recently. Apparently, a critical mass causing it to sink was reached somewhere in the past.
Pumping water from under the cities is causing wash out of sediment.
Um, no. Pumping water can cause sinking, but just because the water is gone, so he land sinks into the empty area. They're not pumping up sand. But that's not really the case on the coasts, as sea water flows in to replace the pumped out freshwater.
Dams up stream stop sediment replenishment and change underground water flow patterns..
I'd hate to drink the sediment-filled groundwater pumped into the sinking buildings of Billy's reality.