The 1931 Paris Colonial Exposition in Bois de Vincennes necessitated the completion of Line 8 for its opening. The extension of the line to Porte de Charenton, including the construction of seventeen 105-metre stations, was agreed on 25 March 1924 and work began in 1928. The configuration of Lines 8 and 9, under the Grands Boulevards in wet, unstable ground, drove the decision to extend the line on two levels. The Line 8 stations are on the upper level: two half-stations, separated by a central supporting wall to ensure stability. The 7.8-km extension was completed in March 1931 and opened to the public on 5 May, ending at Porte de Charenton. The line now included thirty-three stations between Auteuil and Porte de Charenton.