How many gallons are there in one square mile?

Gallons and square miles can't be compared; a gallon is a measure of volume, whereas a square mile is a measure of area. But let's try to make sense of the apparently nonsense question. What if the question poster had asked, "How many gallons are there in a one-square-mile puddle of water that is a quarter of an inch deep?" That calculation is non-trivial and may yield an answer that is surprising. Here are some units and conversions that may be helpful: 1 square mile = 27878400 square feet
1 inch = 0.083333 foot
1 cubic foot = 1728 cubic inches
1 gallon = 231 cubic inches
If you did the arithmetic right, that rather large puddle contains 4,344,685.7 gallons of water. (The proof is left to the reader.)