  We show that extremal Kerr black holes are sensitive probes of new physics.
Stringy or quantum corrections to general relativity are expected to generate
higher-curvature terms in the gravitational action. We show that in the
presence of these terms, asymptotically flat extremal rotating black holes have
curvature singularities on their horizon. Furthermore, near-extremal black
holes can have large yet finite tidal forces for infalling observers. In
addition, we consider five-dimensional extremal charged black holes and show
that higher-curvature terms can have a large effect on the horizon geometry.
