Extremal black holes are known to develop a horizon instability in the presence of scalar matter. In this talk, I will discuss how this picture extends to charged perturbations, where the nonlinearities of both Maxwell’s equations and Einstein’s equations lead to qualitatively new behavior. In particular, I will give numerical evidence that dynamical extremal black holes act as universal threshold solutions of the Einstein-Maxwell-scalar field system. These threshold solutions can be understood from the perspective of critical phenomena, with horizon instabilities emerging in the appropriate limit. I will then present numerical results on the existence of arbitrarily large curvatures in the (near-)extremal interior. This curvature may be visible from future null infinity in spacetimes without a black hole region. Based on arXiv:2602.11256.