We're sitting on a lot of energy.

A comprehensive new MIT-led study of the potential for geothermal energy within the United States has found that mining the huge amounts of heat that reside as stored thermal energy in the Earth's hard rock crust could supply a substantial portion of the electricity the United States will need in the future, probably at competitive prices and with minimal environmental impact. . .

The study shows that drilling several wells to reach hot rock and connecting them to a fractured rock region that has been stimulated to let water flow through it creates a heat-exchanger that can produce large amounts of hot water or steam to run electric generators at the surface. Unlike conventional fossil-fuel power plants that burn coal, natural gas or oil, no fuel would be required. And unlike wind and solar systems, a geothermal plant works night and day, offering a non-interruptible source of electric power. . .

Toksöz added that the electricity produced annually by geothermal energy systems now in use in the United States at sites in California, Hawaii, Utah and Nevada is comparable to that produced by solar and wind power combined. And the potential is far greater still, since hot rocks below the surface are available in most parts of the United States.

Even in the most promising areas, however, drilling must reach depths of 5,000 feet or more in the west, and much deeper in the eastern United States. Still, "the possibility of drilling into these rocks, fracturing them and pumping water in to produce steam has already been shown to be feasible," Toksöz said.

So, why is it so hot down there?

The planet's internal heat was originally generated during its accretion, due to gravitational binding energy, and since then additional heat has continued to be generated by the radioactive decay of elements such as uranium, thorium, and potassium. . .

Much of the heat is believed to be created by decay of naturally radioactive elements. An estimated 45 to 85 percent of the heat escaping from the Earth originates from radioactive decay of elements concentrated in the crust.

In a sense we don't need to build nuclear reactors since we have a very big one underground. Dirt is a decent insulator though, so we have to dig or drill down to the heat.

There are many small scale applications of the principles that are useful down to the level of individual residences. These don't tap deep, hot rocks that make steam, they just leverage the difference between the air and the soil a few feet down. It's warmer in winter, cooler in summer, and there are a variety of ways to exploit the difference. Think heat pumps or even just make-up air.

It seems that the knowledge and technologies we have developed drilling for oil could be applied to enhanced geothermal systems (EGS). Even the deep sea drilling skills might be useful since there are some nice hot spots below sea. This might be attractive to energy companies since the business might be much like what they are used to doing and are staffed and equipped to do. As fossil fuels decline they might be able to shift over to EGS.

With energy we can pretty much make the things we need, even fuels.