Cement

TreeHugger has an article about the pollution generated in order to produce cement, probably the most essential building material in use today.

There is some advance in making EcoCements, but I don't see those building blocks at my local home center yet for non load bearing projects.

So, all those wonderful, low heat and emissions underground houses made entirely from poured structural concrete turn out to not be a complete win, due to the embodied energy and pollution in creating the material that makes the building possible. Further proof that nothing is free, and all environmental decisions are a trade-off.