  The death of massive stars is believed to involve aspheric explosions
initiated by the collapse of an iron core. The specifics of how these
catastrophic explosions proceed remain uncertain due, in part, to limited
observational constraints on various processes that can introduce asymmetries
deep inside the star. Here we present near-infrared observations of the young
Milky Way supernova remnant Cassiopeia A, descendant of a type IIb
core-collapse explosion, and a three-dimensional map of its interior, unshocked
ejecta. The remnant's interior has a bubble-like morphology that smoothly
connects to and helps explain the multi-ringed structures seen in the remnant's
bright reverse shocked main shell of expanding debris. This internal structure
may have originated from turbulent mixing processes that encouraged the
development of outwardly expanding plumes of radioactive 56Ni-rich ejecta. If
this is true, substantial amounts of its decay product, 56Fe, may still reside
in these interior cavities.
