The main continent is Thora, extending also far into the tropical zone of Strabol and the arctic zone of Karbol -- possibly as far as the south pole.
The other impediment to communication, Amtor's quirky cartography, stems from the inhabitants' bizarre cosmology, which at least in the southern hemisphere where Napier lands holds that the world is a flat disc floating on a burning sea of lava, with a rim of ice and a center of fire.