  The magnitude of the August 2003 blackout affecting the United States has put
the challenges of energy transmission and distribution into limelight. Despite
all the interest and concerted effort, the complexity and interconnectivity of
the electric infrastructure have so far precluded us from understanding why
certain events happened. In this paper we study the power grid from a network
perspective and determine its ability to transfer power between generators and
consumers when certain nodes are disrupted. We find that the power grid is
robust to most perturbations, yet disturbances affecting key transmision
substations greatly reduce its ability to function. We emphasize that the
global properties of the underlying network must be understood as they greatly
affect local behavior.
