Oh, and just FYI?
There's something called the "balance of probabilities." Ever heard of it? It's the lowest level of proof required, which is above 50% chance of it being true. The standard is met if the proposition is more likely to be true than not true.
While it's literally impossible to deny anything if you care to argue about it, the fact is that we don't rely on methods like that--we rely on probability. So it's certainly possible that gravity doesn't exist, the earth is flat, and any number of other theories are true. However, when you throw Occam's Razor into the mix (the simplest solution is generally the best one), it becomes obvious that the explanation given by science--which has been repeatedly verified by sources the world over--is far more compatible with the Razor than any other explanation. Sure, it's possible that every government in the world has been involved in a massive conspiracy for hundreds upon hundreds of years. It's just not very likely at all.
Furthermore, the "Round Earth Theorists" have provided far more evidence to support their claims than any "Flat Earth Theorist" has. Burden of proof means that if you're going to suggest a theory or state a claim, you must provide concrete evidence to support it. Merely claiming "well you can't disprove it" is not sufficient, for obvious reasons. Case A is not proven simply because Case 'Not A' cannot be proven. "A is true because Not A is false" simply doesn't cut it. The sky is green because it is not red. Ludicrous, correct? However, that's the same type of logic being used in "The Earth is flat because you can't definitively prove it is round."
And once again, Occam's Razor:
entia non sunt multiplicanda praeter necessitatem,
"enteties should not be multiplied beyond necessity."
When competing theories are equal in other respects, the Razor says to select the theory that relies on the fewest assumptions and fewest postulates. A global conspiracy and thousands of people guarding a gigantic ice wall certainly relies on far more assumptions and postulates than "the Earth is round; here are pictures and data collected from experiments and observational studies to support this claim."
And finally, the less reasonable a statement seems, the more proof it requires. Considering the fact that several "Flat Earth Theorists" have admitted that they believe that the moon and other celestial bodies are indeed round, I find it odd that one planet would be indescribably flat and rely on completely different mechanical laws and properties than its neighbors. That is certainly less reasonable than the Earth being similar to neighboring celestial bodies--and therefore, requires a much higher level of proof. A fantastic example of this is cold fusion. It is currently not accepted as possible, simply because its veracity would call into conflict many other tested and accepted theories of nuclear physics.
:)