  It is widely believed that asphericity in the explosion is the crucial
ingredient leading to successful core-collapse (CC) supernovae. However, direct
observational evidence for the explosion geometry and for the connection with
the progenitor properties are still missing. Based on the thus-far largest
late-phase spectroscopic sample of stripped-envelope CC supernovae, we
demonstrate that about half of the explosions exhibit a substantial deviation
from sphericity. For these aspherical CC supernovae, the spatial distributions
of the oxygen-burning ash and the unburnt oxygen, as traced by the profiles of
[Ca II] {\lambda}{\lambda}7291,7323 and [O i] {\lambda}{\lambda}6300,6363
emissions, respectively, appear to be anticorrelated, which can be explained if
the explosion is bipolar and the oxygen-rich material burnt into two detached
iron-rich bubbles. Our combined analysis of the explosion geometry and the
progenitor mass further suggests that the degree of asphericity grows with the
mass of the carbon-oxygen core, which may be used to guide state-of-the-art
simulations of CC supernova explosions.
