On January 28, 1887, French engineer Gustave Eiffel started construction work of his famous eponymous Tower in Paris. Finished 26 months later in March 1889 as the entrance arch to the 1889 World's Fair, it has become both a global cultural icon of France and one of the most recognizable structures in the world. Until today, the tower is the tallest structure in Paris and the most-visited paid monument in the world; 7.1 million people ascended it in 2011.
On August 21, 1911 during intensive repair and renovation work the Louvre Museum realized that Leonardo Da Vinci's most famous painting, the Mona Lisa, was stolen.
On August 8, 1900 German mathematician David Hilbert gave a speech at the Paris conference of the International Congress of Mathematicians, at the Sorbonne, where he presented 10 mathematical Problems (out of a list of 23) all unsolved at the time, and several of them were very influential for 20th century mathematics.