If you point your finger straight down, from anywhere you sit, the center of the earth is 4000 miles in that direction.
ALL of the water on earth is in the top few miles of that, the rest is rock, and maybe a molten iron core.
As you may have noticed, rock is more dense than water. Now, on geologic and cosmologic time scales, rock is actually fluid, which is why planets (and the moon) are spherical. If the rock could not flow they would be quite lumpy.
In the high pressures and temperatures the rock is a fluid that is more dense than water, so the water floats to the top. This is just like oil or gasoline floating on water.
The rock under the pine barrens is porous and the water is in the pores. There is not a vast underground lake like Lake Superior under your feet in some kind of cavern. Four trillion gallons of water is like someone passing gas in a tornado compared to the mass of the earth.