  In this essay we show that an uncharged black-hole moving superluminally in a
transparent dielectric medium violates Hawking's area theorem. The violation is
overcome through the emission of radiation. Since modes cannot emerge from the
black hole itself, this radiation must originate from a collective effect in
the medium, in complete analogy with the Vavilov-Cherenkov effect. However,
because the black-hole is uncharged, the emission mechanism must be different.
We discuss the physical origin of the effect and obtain a Newtonian estimative.
Then we obtain the appropriate equations in the relativistic case and show that
the field which is radiated away is a combination of gravitational and
electromagnetic degrees of freedom. Possible astrophysical relevance for the
detection of primordial black-holes and binary systems is discussed.
