In our part of the world the Ogallala Aquifer is really important, and has been written about extensively. It is often touted (sometimes accurately and sometimes not) as the largest, the most heavily developed and/or the most stressed aquifer - in the northern hemisphere, the US and/or the world.
However, when it comes to size and volume of water, it's the Guarani Aquifer that clearly sets the bar. This expansive aquifer is said to cover 1.2 million square kilometers, but quite a bit of its extent has yet to be verified - it might be even bigger. It underlies parts of Brazil, Paraguay, Uruguay and Argentina in South America, with Brazil having the lion's share (61.6%) of it. Argentina, Paraguay and Uruguay have 21%, 8% and 3.3% respectively. According to one published synopsis (Cassuto and Sampaio) at the rate current usage the aquifer will last for another 2,000 years. Wikipedia is a bit more generous, claiming that at the worlds current population (6.9 billion) the Guarani could supply the worlds drinking water needs for 1,600 years.
Well, what to do with the Guarani? Not long ago the 4 countries claimed sovereign ownership of the aquifer and have been working together to develop a transboundary agreement. The process is called GAS, for Guarani Aquifer System. While all four countries have signed the GAS agreement, not all of the Countries have ratified it yet. And not everyone is satisfied with the agreement, which according to some, is more fluff than substance and as such, will be an ineffective development and management directive. I say not to worry, for if they screw this up, we've developed the LEMA process they can use to correct things later. Anyway, now you know a bit about the Guarani Aquifer.