Deep in the Hub, deeper even than the archives, and the cells where aliens displaced by the Rift live out their lives until they can be returned to where they belong, there are rooms and tunnels that have never been used, space enough that every person now living in Cardiff could probably hide out there in relative safety in the event of invasion or some other disaster.
Ianto likes it down there. He spent days mapping the tunnels when he first arrived, finding previously unknown entrances. That’s how he got Lisa into the Hub.
Harkness still doesn’t have a clue.