What is an antipode?
An antipode is the exact point on Earth directly opposite another point. If you could draw a perfectly straight line through the center of the planet, the place where it exits is the antipode.
How it is calculated
The latitude flips north to south or south to north. Longitude moves 180 degrees around the planet. In code, that means latitude becomes negative, and longitude shifts by 180 degrees while staying inside the -180 to 180 range.
Why so many antipodes are ocean
Most of Earth's surface is water, and land is unevenly distributed between hemispheres. That means many cities have antipodes in open water, while a few regions create rare land-to-land pairs.