Geothermal power

100 gigawatts of geothermal power for a $100 $1 billion investment, with no associated carbon load and no radioactive waste storage problem, sounds pretty damn good to me.  According to this article, we'd only need to tap 0.0007% of the shallow geothermal heat beneath the United States (where "shallow" is 3-4 miles) to meet the US demand for electrical power.  Double that, and we could easily convert most transportation to electric traction without wondering where the power would come from.

Now this is sustainable energy.  Individual sites may cool and need to recover, but over the long run, geothermal heat will outlast the human race.  Even if we manage to somehow exhaust all the shallow geothermal sources, it'll take us long enough that by that time we may be ready for a mantle tap.

Edit:

I read "$100 million to $1 billion" in the article, decided to take the high number to be safe, and fluently typo'd "$100 billion".  Corrected above now.  That makes the construction investment on the order of one cent per watt.

(Article pointer from mrmeval)