The elevation map of Victoria crater was generated using multiple observations from HiRISE imaging off-track by several degrees to generate ultra-long baseline stereo.

You couldn't do the same with the caves as a black hole has no features to co-register between mutiple stereo observations.

If there were areas taht were volcanically hot - we'd know. THEMIS would have gone off the scale with the sort of heat involved with a volcano.

Long lived large expanses of open water are not possible - it would be a boiling, steaming mess. A collapsed lava-tube is the simplest explanation for the multiple cave entrances that have been observed.

Doug