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This is a great question, and one I have been contemplating, as well.

My opinion is that it would be most cost-effective to excavate the site, install a monolithic concrete dome and cover with soil.

Here is a link that may be useful.

http://static.monolithic.com/plan-design/belowgrad...

Anything you build should be approved by the building department in your area. They will probably require engineering etc. If you dig a tunnel, there may be a geological study involved.