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The term plunging region is used for the region around a black hole where orbits must be unstable, which is within the ISCO-radius, i.e., the radius of the innermost stable circular orbit (ISCO). Matter within this region would generally fall into (plunge into) the black hole. For the simple case of a black hole with no rotation:
RISCO = 6GM/c² = 3RS
A means of detecting the existence of such a plunging region is based upon spectra: a black-body spectrum is expected to reflect the temperature of a black hole's accretion disk. Substantial within the ISCO-radius would result in detectable differences to the spectrum. In 2024, such detection of a plunging region was announced.