Line 7bis was originally part of Line 7 before 1967, to improve service on the main line. It is the second-shortest line after Line 3bis, but it is also one of the most interesting. It cuts through the Buttes-Chaumont area of Paris deep underground, passing through enormous caverns left behind by old gypsum mines. It currently uses MF88 stock, which is strangely more modern than the stock used on most other lines.