First off, great job! super addicting game. I love the 8-bit style graphics, made for a very nostalgic gaming experience.
Now for some science.
I want to believe your 350m estimate, but it seems very far fetched. If the diameter of earth is ~12,700km (~8,000 miles), this does not seem right. This is assuming you started in Argentina or some other Antipode of china. Even traveling on the surface, at 350 Km from the center of china, you're still in china. So if you dug a hole, in china, for 1000 times the distance in the game, you'd still be in china and only about a quarter of the way to the other side of earth.
I would love to see the math behind your estimate. I'm really not trying to be rude, I'm just very curious as to how you arrived at that figure, since it is orders of magnitude off the estimate I have made.
Thanks again, you did an amazing job!