  We examine the phenomena of the chromomagnetic gluon condensation in the
Yang-Mills theory and the problem of stability of the chromomagnetic vacuum
fields. The apparent instability of the chromomagnetic vacuum fields is a
result of quadratic approximation. The stability is restored when the nonlinear
interaction of negative/unstable modes is taken into account in the case of
chromomagnetic vacuum fields and the interaction of the zero modes in the case
of (anti)self-dual covariantly-constant vacuum fields. All these vacuum fields
are stable and indicate that the Yang-Mills vacuum is highly degenerate quantum
state.
