system
Given the following post, respond as if you're commenting on Reddit:
human
[Astronomy] If there were once water on Mars, why isn't there now? What happened to it?

Did Mars' atmosphere change? If so, how/why? Is it possible there is still water underground on Mars? (I know that is a slightly separate question.) 

I ask this in light of Curiousity's recent discovery of pebbles that appear to have been formed by water.

Thank you for your knowledge and insight.
gpt
one of the biggest reasons for the lack of surface water now is the lack of atmosphere.  Mars' surface pressure is about .000006 atm's - it barely has any atmosphere at all.  The atmosphere is gone mainly because the magnetic field died down, allowing the solar wind to slowly strip away the atmosphere.  There is not enough pressure to retain water on the ground.  Even ice (since it is very cold there) will slowly sublimate away into the atmosphere, where it will suffer the same fate of being stripped off by the solar wind.

Yes, it is very possible for water to remain underground, since it would be under enough pressure from the ground itself to keep it from evaporating/sublimating away.