The Antipodes and Inhabitable Zones
If the earth is a globe, somewhere beneath your feet other people stand upright, their down our up — across a torrid belt no traveler can cross.
Once the Greeks accepted that the earth was a sphere, a startling consequence followed: the surface must divide into climatic belts, with a scorching equatorial zone presumed too hot to cross and temperate bands on either side. This raised the haunting possibility of "antipodeans" — people living on the far side of the globe with their feet pointed opposite to ours, forever sealed off from the known inhabited world by the impassable torrid zone. Plato had already argued that on a sphere there is no absolute "up" or "down," dissolving the objection that such people would fall off, while later geographers like Crates of Mallus mapped the earth into multiple inhabited patches. Cicero gave the idea its most famous expression in the Dream of Scipio, where the cosmos is viewed from above and the human oikoumenē shrinks to a few scattered specks.
Where this idea shows up
1 Greek sourceThe works that discuss this idea don't yet have curated places we can drop on the map.