  We present a new mechanism to deplete the energy density of the QCD axion,
making decay constants as high as $f_a \simeq 10^{17}\,\rm{GeV}$ viable for
generic initial conditions. In our setup, the axion couples to a massless dark
photon with a coupling that is moderately stronger than the axion coupling to
gluons. Dark photons are produced copiously through a tachyonic instability
when the axion field starts oscillating, and an exponential suppression of the
axion density can be achieved. For a large part of the parameter space this
dark radiation component of the universe can be observable in upcoming CMB
experiments. Such dynamical depletion of the axion density ameliorates the
isocurvature bound on the scale of inflation. The depletion also amplifies the
power spectrum at scales that enter the horizon before particle production
begins, potentially leading to axion miniclusters.
