Ideas for a cheap storm shelter/root cellar?

I've got access to a back hoe that can hollow out an area in a 6' hill behind my house, but what then?  I'm on a budget, so it needs to be cheap, cheap, cheap.  I have access to a lot of old tires, so I thought about using them kinda like the whole "earthship" idea, but I worry about off gassing.  Also, how would you support the roof?

on NPR they recently had a story of a family that survived the alabama tornados in a buried school bus.  Guy said his brother took shelter in his buried van.  Seems like a good idea.  One of the root cellar books I saw recently showed an old bread truck or something.  Food for thought anyhow.

earthbags, just make the walls curved.  very easy construction.  Fill old feed sacks with the earth you dug out and tamp them to form a solid brick.  You fill them right in place on the wall.

Also, check out Mike Oehler's the $50 and up underground house book.

We  have a 8 foot diameter calvert  22 feet long set into the hillside.It was free for the hauling.The back wall is reclaimed  concrete reinforced block.The front entry is stone covered.We need to add one more door three more feet in so it has a to eliminate the cold rush we get in the winter... It is wired and lit inside.On top of it we have an 18ft long by 9 ft wide tomato hoop house.We do not have much flat area at our place.We also have one a bit shorter then this set under a road area going to our chicken coop area.
However we have the equipment and trailers to haul and do this ourselves, so it is our time , fuel,and labor.

Mary
from the Happy House teaching gardens
  Rollins ,Montana

I've got my hole pretty much dug and am in the same spot as you. I've been thinking an earthbag dome, there's an article in Mother Earth News about one that is fairly in depth. I just was talking to a friend about what to do and he suggested to check with the local concrete company and see if they have any sewer pieces that were mistakes that they would sell for cheap and then just put a lid with a hatch and ladder to access it.

Just a thought, good luck!

peace