As you'll see in exploring the links below, the earth has a strong magnetic field. Scientists are just beginning to understand how the field is generated, maintained, and how it has changed over time.
For purposes of the thruster project, the magnetic field is important because our understanding of it supports several ideas offered in these pages.
It seems that the magnetic field changes direction from eon to eon. This suggests that if we were to reorient the mantle around the core, it may not be the first time that this has happened.
The magnetic field has a North and South pole, and perhaps they should (but don't) line up with the north and south pole we recognize up here on the crust. Scientists have theories explaining the variance, but I don't find any consensus.
This leaves open the possibility that the crust and mantle were shifted around the core by something. One could imagine that the asteroid that hit the earth and wiped out the dinosaurs might have spun the mantle around the core when it hit. It is possible that our present orientation is the net results of millions of years of asteroid hits, volcanoes exploding, shifting plate tectonics, and other assorted effects. In any case, the magnetic pole is quite a distance from the poles around which we spin today.
http://www.survivalnetwork.org/articles/24.html
http://138.67.1.32/fs_home/tboyd/GP311/MODULES/MAG/NOTES/earth_field.html
If you prefer a more scientific answer as to why the poles don't line up, read this: