Underground cities as a serious method for development.
We humans are pushing Earth to its limits. We are standing on thin ice and our own extinction may not be that far away. We are wasting to the land on Earth and taking what we have for granted.
The pdf linked details pros and cons to making more use of underground tunnels and caves. But I say we go a step further and start to consider developing methods of large scale cave societies.
The pdf tells us that caves are actually shown to deal extremely well with earthquakes so that reduces the risk of cave ins. Also this idea is not so far fetched, already in Australia
http://en.wikipedia.org/wiki/Underground_living people are living underground.
By going underground we can dedicate more land to foresting, agriculture and energy production. Solar energy fields could potentially power underground towns, geo thermal energy could be harnessed and as the pdf outlines there are no temperature changes underground so it is always constant. This opens more land for planting trees and plants to tackle global warming. We can travel underground via something like a maglev train. Rain for water can be accessed as it is now and also we can harness flood waters by creating a system which can train some flood water and store it underground for times of water deficit. Also given potential thermonuclear warfare or even asteroid impact being underground and well supplied isn't a bad thing.