Concrete Canvas

http://www.concretecanvas.org.uk/

Instant semi-permanent structure.
Fill bag with water, activate chemical gas-pack, it inflates and hardens and is ready for use in 12-hours.

Update (1 Feb 2006):
My friend and architect Bob Theis (http://www.bobtheis.net/) raised these important issues:

Clever idea. Two reactions:

1. Hard to imagine a shell that thin being dimensionally stable enough for such a span in compression ( how does it resist local buckling? ). ESPECIALLY when you cover it with earth to gain some insulation ( see below ).

2. The plastic sheet interior would condense ALL the water vapor that hits it when the temperature outside is low ( despite the website claims, the insulation value of the skin should be next to nil ), where it would then freeze in colder situations. So insulation and ventilation would be serious habitability issues.