Once upon a time wise men believed that it is natural tendency of bodies to fall down. The discovery that earth is like a sphere and “up” direction is not constant everywhere probably became a constant enigma. As a supplement to the theory of up and down the wise men then believed that if you cross the equator you will fall down to the bottom of universe. Probably they thought earth to be a hemispherical body or further smaller part of sphere.
There were always not so wise men in history. They crossed the equator at will either on foot in africa and america or through ships in africa, america and indochina to austraila. It is quite likely that then wise men did not come across the stories told by these people or they did not believe them. So the idea of up and down remained for quite some time even till medieval times.
We come across the up-down theory in large number of myths. All these myths require earth to be supported. Usually north is considered up and south as down in most of the myths.
People in Kerala believe that the king Bali comes up (north) and visit the people of kerala every year. This tale signifies that the patalalok was somewhere in south.