Mars. The water's still there.
I had a 'theory' a long time ago about where Mars' water went. The Hellas basin is an enormous impact crater 4400 miles across and 5 miles deep. The energy of the impact was restrained to Mars and dissipated through reverberation and heat. This may have caused a type of large scale liquefaction. The water seeped into the ground like a puddle in a freshly tilled garden.
Plausible?
New study changes everything.