We thank R. Burk for discussions that led the authors to focus on the interesting new scientific principles governing the catastrophic collapse of coupled networks. We also thank V. Rosato for providing the Italy 2003 blackout data. S.V.B. thanks the Office of Academic Affairs of Yeshiva University for funding the Yeshiva University high-performance computer cluster and acknowledges the partial support of this research through the Dr. Bernard W. Gamson Computational Science Center at Yeshiva College. S.V.B., G.P. and H.E.S. thank the Office of Naval Research for support. S.H. thanks the European EPIWORK project and the Israel Science Foundation for financial support. We thank E. Leicht and R. de Souza for discussing their unpublished work with us.

Author Contributions S.V.B., R.P., G.P., H.E.S. and S.H. all participated equally in the conceptual design of the model, the theoretical analysis, the computer simulations and the writing of the paper.