re: Don't get carried away......
Caves - probably.
Thermal vents - maybe.
Underground rivers - I think not. The water most likely existed a few MILLION/BILLION years ago.
Symbiotic life - I think not. When you factor in the extreme temperature changes and the massive amounts of (life killing) solar/space radiation absorbed by Mars, the prospects for life fall off dramatically.
Sites that will tantilize scientists for decades - no doubt.
Vince