In the most popular Flat Earth Model, the outer edge of the Earth is bounded by an 'Ice Wall'. This wall prevents the oceans from spilling over the side of the Earth, and may perform the same function for the atmolayer. The exact size of the Ice Wall varies between different Flat Earth Models.
The traditional view is that the Ice Wall rises approximately 150 feet above sea level, preventing the ocean from spilling over the edge of the Earth. In this model, the atmolayer is either contained by another means or universal. This view of the Ice Wall is generally agreed to correspond to the coastline of Antarctica in the Round Earth Model.
It is unknown whether the ice extends outward forever or there is an actual boundary to the plane upon which the Earth resides.
Others believe the Ice Wall to be much larger. In this model, there is an impenetrable boundary of ice further outward around the edges of the Earth, generally estimated to be 40,000 - 50,000 feet high to hold the atmolayer in place. This Ice Wall is considered to mark the outer edge of the earth by those who believe it exists, although what truly lies beyond it remains a matter of speculation.
Samuel Rowbotham had this to say about the Ice Wall:
How far the ice extends; how it terminates; and what exists beyond it, are questions to which no present human experience can reply. All we at present know is, that snow and hail, howling winds, and indescribable storms and hurricanes prevail; and that in every direction "human ingress is barred by unsealed escarpments of perpetual ice," extending farther than eye or telescope can penetrate, and becoming lost in gloom and darkness.
It should be noted that, in both views, the Ice Wall is a naturally formed structure, a mountain range merely covered in ice and snow. Thus, it is irregular in shape, size, and appearance. It is a wall only in the sense that it walls the earth’s oceans.