I wonder what happened to all the water on Mars. Unless there were lakes of liquid CO2 or methane in the past, pictures and measurements of Mars' geology strong

I wonder what happened to all the water on Mars. Unless there were lakes of liquid CO2 or methane in the past, pictures and measurements of Mars' geology strongly indicate that there was a huge ocean and lots of river systems and/or glaciers in the past. The most likely-seeming explanation is that it sublimated to gas and was gradually kicked out of the atmosphere by the solar wind, but there are plenty of experiments we could do to figure out if this was really the case. If Mars still retains most of its water in frozen form (perhaps trapped under a layer of rock in the northern hemisphere), there is a possibility that the planet could at some point be terraformed. Or at least that it would be much easier than expected to set up a self-sufficient settlement there. There is just so much about the planet we don't know. Sending a lot more probes would be a great way to start exploring further and figure out the answers.