  We stack maps of the thermal Sunyaev-Zel'dovich effect produced by the Planck
Collaboration around the centers of cosmic voids defined by the distribution of
galaxies in the CMASS sample of the Baryon Oscillation Spectroscopic Survey,
scaled by the void effective radii. We report a first detection of the
associated cross-correlation at the $3.4\sigma$ level: voids are
under-pressured relative to the cosmic mean. We compare the measured
Compton-$y$ profile around voids with a model based solely on the spatial
modulation of halo abundance with environmental density. The amplitude of the
detected signal is marginally lower than predicted by an overall amplitude
$\alpha_v=0.67\pm0.2$. We discuss the possible interpretations of this
measurement in terms of modelling uncertainties, excess pressure in low-mass
halos, or non-local heating mechanisms.
