  We study geometry of the ring of integers $O_K$ of a number field $K$.
Namely, it is proved that the inclusion $\mathbf{Z}\subset O_K$ defines a
covering of the Riemann sphere $\mathbf{C}P^1$ ramified over the points
$\{0,1,\infty\}$. Our approach is based on the notion of a Serre $C^*$-algebra.
As an application, a new proof of the Belyi Theorem is given.
