If you were to walk by this building at 145, rue La Fayette you'd probably never notice anything special about it.
Upon closer inspection you might notice that some windows are always left open and that the building is always dark. Maybe it is an abandoned building? But for the real reason you have to look at the building from a different angle... from above.
It turns out that the entire building is fake. It is a large ventilation shaft for the Paris Metro system. Instead of installing an ugly pipe the city just built a fake facade so it would blend into the surroundings.