The Polányi Tunnel footbridge

You enter the Polányi footbridge (pronounced [ˌpɔˈlɑːɲi]) at the Batthyány Square (pronounced [ˈbɒcːaːɲi]) quay, walk a 100 metres or so below the Danube river bed, and re-emerge on the Pest side a few hundred meters from Budapest Technocratic Head Offices (see earlier post for a bit more detail) - that is, when the tunnel isn't undergoing one of those frustrating re-constructions that seem to be happening more and more often.
(Disclaimer: I made this up wholesale - don't go looking for the underwater footbridge.)

There's neon-lit garish yellow tilework all through the tunnel, and it has two "lanes" for traffic, one for pedestrians, the other for bicycles, baby carriages, skateboards, et cetera, with a wide divider wall that's interspersed with openings running along the middle.
Motorized vehicles understandably banned.
Probably lots of service tunnels - emergency exits that lead into safe bunkers that can withstand water pressure and explosions, should there be any accidents. Drone of air circulation fans audible throughout the tunnel, louder in places.
Faint aroma of floor polish mixed with the catch of the day - takeaway coffee spilled by low-level ministry officials in a hurry, milk, fish, or vegetables dropping off carriages on their way to the Batthyány Square market hall, some decomposing pigeon pulled apart by crows in the relative safety of the tunnel.
Beggars, musician buskers (if there's more than 2, there is no way you can make out any melody as the tunnel acoustics mess up the sound), and usually quite a few security patrols.
CCTV.

What you can use it for

The tunnel is great for foot chases - especially when its under construction, and one or both lanes are barricaded off.
Given the thick divider walls, there must be quite a few niches where CCTV doesn't pick you up, should a mage require a secluded spot for the exchange of illicit goods and/or information.
There's always a chance of an accident, or better yet, a terrorist act that happens to include some of the PCs walking in the tunnel.
Hostage situation.
A mage terrorist cell (they exist!) attempting to infiltrate the Szabadság tér Szövetkezet ([ˈsøvɛtkɛzɛt]) headquarters from within the cover of the tunnel.
The whole site is kind of spooky, and I imagine both the Gauntlet and the Consensus are a bit flakier than usual down there - just remember that you're 15 metres underground, below a whole lot of water.
(Image credit Dominik Martin at Unsplash)