The Flerfers don't understand that they are living in three dimensions. All they need do is rotate their frame of reference by 90°.
The horizon is the point where a line from the eye of the observer meets the surface of the Earth at a tangent. As such, it describes a circle on a sphere with the observer in the center. The radius of the circle, distance to the horizon, is dependent on the height of eye of the observer.
By definition, the horizon is the point where the sphere curves away from them and is lost to sight.
SpyNavy
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)
Mom's basement looks pretty flat. They need a bigger frame of reference.