
It uses a Rocker-Bogie suspension, very cool. I didn't understand how exactly this keeps the robot level at first, until I found some more info and this model (and others):

The "trick" is that both rockers (the large frames/bows) are connected through a differential. Anyway, of course this has been done with LEGO and Mindstorms before. I'll let you google and look on youtube yourself, but two really cool videos of an NXT bot:
And last vid: The real rover (model of the suspension system) in action:
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