  Combining in-depth neutron diffraction and systematic bulk studies, we
discover that the $\sqrt{5}\times\sqrt{5}$ Fe vacancy order with its associated
block antiferromagnetic order is the ground state, with varying occupancy ratio
of the iron 16i and vacancy 4d sites, across the phase-diagram of K$_{\bf
x}$Fe$_{\bf 2-y}$Se$_2$. The orthorhombic order with one of the four Fe sites
vacant appears only at intermediate temperature as a competing phase. The
material experiences an insulator to metal crossover when the
$\sqrt{5}\times\sqrt{5}$ order has highly developed. Superconductivity occurs
in such a metallic phase.
