  In study of muon neutrino disappearance at 810 km, the NOvA experiment finds
flavor mixing of the atmospheric sector to deviate from maximal
($\sin^2\theta_{23} = 0.5$) by 2.6 $\sigma$. The result is in tension with the
295-km baseline measurements of T2K which are consistent with maximal mixing.
We propose that $\theta_{23}$ is in fact maximal, and that the disagreement is
harbinger of environmentally-induced decoherence. The departure from maximal
mixing can be accounted for by an energy-independent decoherence of strength
$\Gamma = (2.3 \pm 1.1) \times 10^{-23}$ GeV.
