  Fast radio bursts (FRBs) are bright millisecond radio bursts at cosmological
distances. Only three FRBs have exhibited extreme activities, such as achieving
a peak event rate $\gtrsim 100$ hr$^{-1}$ or being persistently active. Only
these three among $\sim 50$ known repeating FRBs have circular polarization. We
observed the FRB 20220912A with the Robert C. Byrd Green Bank Telescope (GBT)
at L-band on 24 October 2022 and detected 128 bursts in 1.4 hours,
corresponding to a burst rate of about 90 hr$^{-1}$, which is the highest yet
for FRBs observed by the GBT and makes it the fourth extremely active FRB. The
median energy of the bursts is $4.0\times10^{37}$ erg, close to the
characteristic energy of FRB 20121102A. The average rotation measure (RM) was
$-$0.4 rad m$^{-2}$ with unnoticeable intraday RM change, indicating a likely
clean environment, in contrast to the other three extremely active repeating
FRBs. Most bursts have nearly 100% linear polarization. Approximately 56% of
the bright bursts have circular polarization, the highest such fraction among
all FRBs. A downward drift in frequency and polarization angle swings were
found in our sample. The discovery and characterization of FRB 20220912A
support the view that the downward drift in frequency, polarization angle
swings, and circular polarization are intrinsic to radiation physics, which may
be shared by active repeaters regardless of the environments.
