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Below are five pairs of global cities. What do the sets have in common?
Madrid, Spain / Wellington, New Zealand

Buenos Aires, Argentina / Shanghai, China
Hanoi, Vietnam / La Paz, Bolivia
Johannesburg, South Africa / Honolulu, Hawaii

Show Answer
These cities are Antipodes, i.e. on the diametrically opposite sides of the Earth.
Interestingly only 15% of the Earth's land surface has a land antipode. 
See the Antipodal Map of Earth to see what is opposite what.