Almost nothing if you use already mined out space, that is the attraction. There are a number of places in the Midwest where the limestone was mined from underground leaving supported underground space under places such as Kansas City and Springfield, which have been turned into factories and offices. Because of the insulation and isolation it had advantages in certain industries, and has a very stable temperature. And it might be possible to use other mined out space - some of the metal mines, for example, leave rather large cavities.

The fact that algae feed on carbon dioxide makes an underground coal mine – like UMR’s Experimental Mine – the perfect incubator. According to Summers, the United States has a lot of unused space underground. “In underground coal mines, temperatures are easy to keep constant,” he says, “even at the higher temperatures that algae prefer.”

Summers also realized that, under the surface, the algae would not be vulnerable to infestation and could be grown in three dimensions. “Soybeans and other crops grow on the surface in two dimensions,” Summers says, flattening his hand in the air to illustrate a point about volume. “Algae isn’t a scarce resource, by any means. It breeds so rapidly under the right conditions – you could fill a whole mine eventually from one vial. And algae don't really need all of the light that comes from the sun. If the electrical engineers can provide a good source of light underground, you’ve solved some big problems.”

You are never going to replace the sun efficiently. Electrical engineers understand this - their crowning achievement was the spread of 1% efficient light bulbs, currently being replaced by 10% efficient bulbs by the green movement. Algae do not live on CO2 - they live on sunlight, their energy source - which they store by building CO2 up into carbohydrates. They live on great scads of it, a kilowatt per square meter peak (around 200 watts average). A thin layer of slime might not use all of that sunlight - and so algae have evolved mechanisms to rise to the top when they need energy, and sink when they're full; Even if they hadn't, simple water pumps can cycle a tank very, very efficiently. Even largescale agricultural processes like cheesemaking do it on a few watts of handpower, often.

Plant biofuels exist to *HARVEST SUNLIGHT.* Sticking them underground is utterly counterproductive.

The United States has a great amount of land *aboveground* that it doesn't use - go find an alkali flat, for the love of Bob.

I agree with Squalish, there is not even a theoretical possibility that underground growing will be energy positive. An EROEI of about 0.1 would be an upper limit based primarily on the lighting inefficiency. You have to wonder what is going on in peoples minds when the think of such projects. It makes coal to liquids look green.