True...Iworry about its shock absorption properties in the earth's crust.plate tectonics and all. But the good news is that kudzu will never run out; sewage will never run out. Food packing waste will never run out. We aren't using what is at hand like we should be. But I am positive that they have patents and engines just waiting to run on whatever. That doesn't mean we wouldn't have a world-wide depression or war waiting for them to be built...but I am sure they are up and running somewhere.. mg