Theorized Torchwood Hub overlaid on Cardiff Map with exits and tunnels marked.
My apologies that it’s small. I didn’t want to expand the screencap and risk distorting it.
Disclaimers:
This map is composed of pieces of documentation from The Torchwood Archives book, info from the series, Google Maps, and a lot of guesswork.
The area directly below the Medbay/Morgue is the original roundhouse for the Torchwood Train System as well as the hub of Cold Storage, as bodies and other items can be moved from the Medbay to Cold Storage by elevator. Because the area of Cold Storage that we see is clearly the rail roundhouse, I mapped London and Glasgow in relation to Cardiff Bay and centered the tunnels on the Medbay area.
These tunnels were damaged by the earthquake on June 27, 1909. There’s never any mention of whether any part of them is still accessible now. They certainly aren’t accessible from the roundhouse, as each of the tunnel entrances has been clearly filled in with cadaver storage.
Hub was oriented underneath Roald Dahl Plass based on the location of the Rift Manipulator inside the Millennium Tower and where the Invisible Lift is placed on the sidewalk relative to the Millennium Tower.
The Hub exit leading to the car park where the SUV is parked is listed in the Torchwood Archive as being ‘at the back of the Weapons Storage area.’ The car park itself is under the Millennium Centre, and I chose a location for said car park close to the road.
The Torchwood Archive also lists the elevator behind the Jolyons Hotel. I connected it to the tunnel leading to the Tourist Center Entrance, because it makes sense.
The Torchwood Archives could easily continue on underneath Cardiff Bay to the East or West. It’s mentioned that Roath Basin (not pictured) could be used to flood the Archives should there be a need to destroy them. So it’s not much of a stretch to think that the Archives themselves could take up two or three city blocks underneath Cardiff Bay. Interestingly enough, none of the maps show any kind of underground transit other than the ones marked*.
*I’m not a Cardiff native. Google Maps could be made of lies.