  While extreme black hole spacetimes with smooth horizons are known at the
level of mathematics, we argue that the horizons of physical extreme black
holes are effectively singular. Test particles encounter a singularity the
moment they cross the horizon, and only objects with significant back-reaction
can fall across a smooth (now non-extreme) horizon. As a result, classical
interior solutions for extreme black holes are theoretical fictions that need
not be reproduced by any quantum mechanical model. This observation suggests
that significant quantum effects might be visible outside extreme or nearly
extreme black holes. It also suggests that the microphysics of such black holes
may be very different from that of their Schwarzschild cousins.
