So this isn’t a space that you can visit but it is a space that you can see. And it’s definitely dark.
In 1913 the Champ de Mars metro station was opened on the line 8, in between la Motte Piquet and Ecole Militaire stations. Its public entrance is at the south-west corner of the gardens in front of the Eiffel Tower. Sadly it was closed down, along with a number of other stations (including Arsenal and St Martin) because of its proximity to other stops on the line.
However, the space was pretty much just bricked up. Beyond the boarded up entrance, still visible on the pavements of Boulevard de la Motte Picquet, sit ghost-like platforms, covered in graffiti. The station now acts as a large extraction vent, sucking musty air out of the tunnels to keep the system fresh.
If you’re travelling through this section of line 8, be sure to look carefully out of the windows. It’s a ‘don’t blink or you’ll miss it’ visit but you might just catch a glimpse of the abandoned platforms.