imagine , if you can , that you're standing in the middle on top of a very large balloon with someone , and they start walking off away from you , once they get to where the balloon starts to curve you'll see less of them , now lets say they reached the circumference of the balloon from where you're standing you won't be able to see them
now imagine if the balloon was punctured , and when it fully deflated onto the ground , and you were still standing where you were originally , and the other person was still at the circumference , you'd be able to see them because the balloon is flat
pump the balloon back up , and voila you can't see them ,