and why is there still a little bit in oases
In: 8
When deserts used to be wet, they had lakes and rivers. Rivers carried most of the water to the ocean. Some of the water seaped into the ground over time and became aquifers — specifically, they became fossil groundwater because they are no longer being recharged with new rainwater, and so as we use them up (e.g. in places like Yemen, or to a lesser extent the only semi-renewable [Ogallala Aquifer in the US](https://en.wikipedia.org/wiki/Ogallala_Aquifer)) they simply go dry over time, unlike renewable aquifers in non-desert places.
But the most important answer to “where the water went” primarily is that the water just stopped coming as climates changed over many many years. sometimes this happened over eons due to continental drift, sometimes it happened somewhat more quickly (i.e. over a period of tens of thousands of years) due to long-term cycles in Earth’s average temperatures and the effects of global lower temperatures (or higher temperatures) on global weather patterns (things like ocean currents and ocean water temperatures having major effects on what areas receive monsoons and what areas never receive rain). So once the rain stopped coming as the climate changed, the water that was there was carried by rivers to the ocean, seaped into the ground, or evaporated and was carried away on the wind.
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