  We present the first statistical study of X-ray cavities in distant clusters
of galaxies (z > 0.3). With the aim of providing further insight into how AGN
feedback operates at higher redshift, we have analysed the Chandra X-ray
observations of the Massive Cluster Survey (MACS) and searched for
surface-brightness depressions associated with the Brightest Cluster Galaxy
(BCG). The MACS sample consists of the most X-ray luminous clusters within 0.3
< z < 0.7 (median L_(X,RASS) = 7*10^44 erg/s), and out of 76 clusters, we find
13 with "clear" cavities and 7 with "potential" cavities (detection rate ~25
per cent). Most of the clusters in which we find cavities have a short central
cooling time below 3 - 5 Gyrs, consistent with the idea that cavities sit
predominantly in cool core clusters. We also find no evidence for evolution in
any of the cavity properties with redshift, up to z~0.6. The cavities of
powerful outbursts are not larger (or smaller) at higher redshift, and are not
able to rise to further (or lesser) distances from the nucleus. The energetics
of these outbursts also remain the same. This suggests that extreme "radio
mode" feedback (L_(mech) > 10^44 erg/s) starts to operate as early as 7 - 8
Gyrs after the Big Bang and shows no sign of evolution since then. In other
words, AGNs lying at the centre of clusters are able to operate at early times
with extreme mechanical powers, and have been operating in such a way for at
least the past 5 Gyrs.
