Few geographic ideas have a biography as strange as the antipodes. What began as a deduction from Greek geometry spent a millennium as a theological controversy, became an insult-tinged nickname for the far side of the British Empire, and survives today in the name of a windswept island group south of New Zealand.
Greek thinkers reasoned their way to the antipodes long before anyone could go looking. Once you accept a spherical Earth — established by the classical period and measured with startling accuracy by Eratosthenes in the 3rd century BC — symmetry practically demands an opposite side, and perhaps opposite inhabitants: antipodes, "those with feet opposite." Crates of Mallus built a globe around 150 BC showing four symmetric landmasses, our known world balanced by hypothetical continents of antipodeans. It was geography by geometry: no one had seen the other side, but the sphere said something ought to be there.
Early Christian writers inherited the sphere but choked on the inhabitants. If all people descend from Adam, and the antipodes are cut off by impassable equatorial heat or endless ocean, how did anyone get there? Lactantius mocked the idea of rain falling upward and men hanging feet-above-head; Augustine argued more carefully that even if the far side held land, no descendants of Adam could have reached it, so tales of antipodean people were "not to be believed." For centuries "do antipodeans exist?" was a genuinely dangerous question tangled in doctrine — a rare case of a geography debate with theological stakes.
When European ships finally crossed the line and nobody fell off, the antipodes shed their controversy and became a place name. From Britain, the literal antipodes are the ocean around New Zealand — so "the Antipodes" settled onto Australia and New Zealand as an affectionate (and occasionally condescending) label, and "antipodean" still means Australasian in British English today.
In 1800 the crew of HMS Reliance sighted a cluster of volcanic islands in the Southern Ocean and named them the Penantipodes — "next to the antipodes" — because they lie nearly opposite Greenwich. The name wore down to the Antipodes Islands. Uninhabited, storm-raked and now a nature reserve, they are the place your tunnel from London or Paris most nearly reaches: our own measurements put Paris's antipode about 271 km from Bollons Island in the group. The islands' main residents are penguins and albatrosses, sublimely indifferent to being the most literal address in the world.
Twenty-three centuries after Crates built his globe, you can settle in seconds what Augustine thought unknowable: type an address and see exactly who, if anyone, stands feet-to-feet with you.