Even in the midst of deserts, water can be found. Much of inland Australia can access water of varying quality from deep underground. The Great Artesian Basin has had so much water taken from it in the past 100 years or so that the water pressure is now dropping. Remedial action is being taken (by capping bores) but it remains to be seen just how effective this is in the longer term.
The water seen here may be a million years old, or more.