Properties of the SU(N_c) Yang-Mills vacuum state

The asymptotic behaviour of the vacuum energy density, ${\bar E}(\theta)$, at $\theta \ra \pm i \infty$ is found out. A new interpretation and a qualitative discussion of the ${\bar E}(\theta)$ behaviour are presented. It is emphasized that the vacuum is doubly degenerate at $\theta=\pi$, and the quark electric string can terminate on the domain wall interpolating between these two vacua. The potential of the monopole field condensing in the Yang-Mills vacuum is obtained.