  The giant gamma-ray flares of the Crab nebula discovered by AGILE and Fermi
observatories came as a surprise and have challenged the existing models of
pulsar wind nebulae. We have carried out an analysis of 10.5 years of Fermi-LAT
observations (Aug 2008 -- Feb 2019) and investigated variability of the Crab
nebula in the 100-300 MeV range. Besides the flares, we found several month
long depressions of the gamma-ray flux and identified several cases of sharp
flux drops, where during one week the flux decreased by an order of magnitude
with respect to its average value. No statistically significant variations of
the nebula flux in the $E>$10 GeV range were found in the data. We discuss
possible implications of the observed gamma-ray flux depressions on the model
of synchrotron emission of the Crab nebula.
