  We prove that, for any finite Blaschke product $w=B(z)$ in the unit disk, the
corresponding Riemann surface over the $w$--plane contains a one-sheeted disk
of the radius $0.5$. Moreover, it contains a unit one-sheeted disk with a
radial slit. We apply this result to obtain a universal sharp lower estimate of
the Bloch seminorm for finite Blaschke products.
