Méridien zéro c. 1667 - 1911
Arago's markers. Look for them! To verify your find e-mail me what they are. Don't post your answer here even if encypted, answers found using the internet rather than visiting with a GPS will not be accepted.
This is the world's old zero longitude. France did not adopt the new 'zero longitude' (Greenwich 1884) until 1911.
Observatoire de Paris
Louis XIV asked Colbert to build an observatory. Plans drawn up by Claude Perrault, it was started on 21. June 1667 and completed in 1672. For scientific reasons the building contains no iron, and for fire safety reasons, contains no wood.
Also visit the Montparnasse Cemetery (close by) and try to find the grave-site of former president/dictator of Mexico before the 1910 Revolution: Gral. Porfirio Diaz!
A great place for a real cache!