From MSNBC:
String theorists say we may live in a 10-dimensional universe, with six of those dimensions rolled up so tightly that we can never see them. So how can you possibly visualize six-dimensional space? This year's top gift for science geeks can help. The 2009 geek-gift competition resulted in a repeat (geek-peat?) of last year's outcome: Andrew Meeusen of Mesa, Ariz., received the most votes once again, this time for suggesting the Calabi-Yau manifold crystal.
So… what the heck is a Calabi-Yau manifold?
That's where extradimensional physics enters the picture: As string-theory fans know all too well, there are inconsistencies between small-scale and large-scale physics that could best be resolved if the universe as we know it has 10 dimensions, including time and the three spatial dimensions with which we're familiar. So what's up with the other six dimensions? Theorists would say we're just not built to perceive those dimensions, perhaps because they folded down to sub-sub-submicroscopic size as the universe took shape. A couple of mathematicians named Eugenio Calabi and Shing-Tung Yau worked out the geometry for how such folded-up extradimensional spaces might behave, and that's how Calabi-Yau manifolds got their name.
More here.