While walking down the street, you’d be forgiven for thinking this Parisian building is no different from the others lining the block. Intricate iron railings guard its windows, and a bright blue door seems to act like a gateway inside. Except unlike its neighboring buildings, this one’s doors never open, and no one’s ever inside to peer out the windows at passersby.

What looks like an ordinary building is actually a facade. It’s basically little more than an elaborate wall used to hide a massive subway air vent. If you look at the building from above (such as via Google Maps) you can see what looks like a gaping hole behind the thin facade.

The sneaky structure was created in the 1980s, when subway developers deemed 145 Rue Lafayette the prime spot to install a vent for the tunnel they were building below. Though they demolished the structure that originally stood in their way, the builders kept the facade to preserve the street’s appearance.

145 Rue Lafayette isn’t the only fake building hiding in plain sight within Paris. There are others, too, waiting for eagle-eyed visitors to spot their deceptive secrets.

In Umberto Eco’s novel Foucault’s Pendulum, the address is mentioned as as one of a few entrances to the underworld used by high-level occult initiates.