  The relation between the large mass of the $\eta'$ and the structure of the
gluon vacuum via the $U_A(1)$ anomaly is discussed. A squeezed gluon vacuum is
considered as an alternative to existing models. Considering Witten's formula
for the $\eta_0$ mass we show that the contact term can give a sizable
contribution and relate it to the physical gluon condensate. The values of the
gluon condensate obtained through this relation are compared with the value by
Shifman, Vainshtein and Zakharov and the recent update values by Narison.
