We've had some interesting questions on being able to see country A from B, and on visibility for miles in every direction.
For a 6 ft tall person, the horizon in a 'flat' area is meant to be about 5km. Obviously the higher you are, the further you can see, if nothing is obscuring your view.
So somewhere on Earth (point A) presumably, you can see all the way, unobscured to Point B, which is further away than any other two visible points on earth.
Where would this line be - that is, between which two points A and B?