Shedworking is a big fan of using straw in a shed build and Uncle Wilco at Shedblog highlights a particular good example of this at readersheds.co.uk where you can find plenty more images. It was built by James Dexter from Norwich who comments:
"May be extended underground at some point to start subterranean 'base'...actually I would be interested to hear if anyone has used their shed as a starting point to dig underground. If the shed is conceived as a 'retreat', then it seems to me that psychologically the next step is to with"draw secretively into the earth itself. No?"