the surface of Mars. It is probably in the form of highly concentrated salt solution, more like the Dead Sea than a freshwater lake or even the ocean. There are algae that live in the Dead Sea today, so there is no reason why microbes could not be living today in this Martian brine a few hundred meters down. "Living on what?" you ask. Living on rocks! Yes, even deep in the Earth, there are bacteria that live on the chemicals found in rocks. They oxidize the ferrous iron to ferric form and thereby obtain energy for life (among other cycles). If it can happen on Earth, why not on Mars?

I strongly suspect that living microbes will be found on Mars when we have landed a rover with a drilling attachment capable of going down a few hundred meters and an automated laboratory capable of analyzing the core samples extracted.

David F Mayer