>>13528051It's not an actual lake, but it's not a visual representation either it's simply an aquifer as in various rocks and sands filled with water and in those locations the thickness of the aquifer is marked out by the color. So in Texas you have parts where there is 100 feet of wet stone while in Nebraska you have spots where there's 1000 feet of wet stone.
Aquifers range in their water content based on the material they are made off, going from 5% in the low end of a usable aquifer to something like 50% with some sands and very porous limestone. Thus you could imagine that instead of being a layer of rock with 1000 feet of saturated rocks in Nebraska it would be equivalent of a surface lake with a depth of 200-400 feet. So there is huge quantities of water down there.
Of course in regions with limestone especially there can be entire caves filled with water which are more of an underground lake in the traditional sense but most of the aquifer isn't like that.
