Louis XVth wished to built a new church to Saint Geneviève, patron saint of Paris, if he was going to recovered from a serious illness, which he did. Soufflot, a young architect, was known only in Lyon, where he buit the Opera. The architect of Louis XV at the time was busy on other sites, so the King finaly decided to take Soufflot. The architecture has been influenced by the Greek Antiquity, and the Dome is bigger than the Vatican one.
During agitated periods of French Revolution and onwards, the Church became a Republican place, and a place to bury famous people like Voltaire, Rousseau, Pierre and Marie Curie, Victor Hugo,... They are 73 at the moment in the crypt.
You can also admire one Pendule de Foucault in the center of the monument (proof of the rotation of the Earth), and a beautiful panoramic view of Paris from the Dôme at certain times of the day.
8 euros for adults. Admission free for under 18.