The Mars Express/Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) has conducted the first-ever subsurface probing of the south polar layered deposits (SPLD) revealing internal structure that is rich in detail. We report on subsurface features detected from a set of orbits that passed over the largest of the south polar reentrants, Chasma Australe, at the edge of the SPLD. We present selected MARSIS overflights to both document the unique observations and gain insight into the possible origin of this feature. We conclude that MARSIS observations clearly reveal new evidence of subsurface structure, such as (1) internal bands, (2) the continuation of the Prometheus basin floor beneath the SPLD to form a prominent basal interface, and (3) the presence of a possible ice-rich layer on the Prometheus basin floor extending to 500-m depth. There is no obvious indication of a present-day aquifer at the head of the chasma and identification of features associated with a past aquifer and water flow is ambiguous. While MARSIS observations may not uniquely solve the chasma origin debate, they do reveal the subsurface region beneath this very interesting feature for the first time.