Galaxy clusters often display large X-ray cavities—regions of reduced X-ray emission carved out of the hot intracluster medium (ICM)—which are typically attributed to jets and outflows from active galactic nuclei (AGN) inflating bubbles of relativistic plasma. However, the energetics and timing of these cavities frequently do not match the observed AGN activity. The mechanical energy required to excavate the cavities often exceeds the luminosity and duty cycle of the central AGN by factors of several, and the cavities appear too large, too numerous, or too old to be explained by the current or recent AGN outburst alone (McNamara & Nulsen 2007; Fabian 2012). Additionally, some clusters show multiple generations of cavities with no corresponding AGN activity visible at those epochs. Lambda-CDM struggles to explain this mismatch because it relies on AGN feedback as the primary mechanism for both cavity formation and preventing runaway cooling in cluster cores, yet the observations suggest that either the AGN are more powerful and long-lived than models predict, or that an additional energy source is at work.