Figures vary year-by-year, but London constantly hovers near the top of Europe's most polluted cities.
Like many of its European counterparts, it sprang up long before the car was invented, leaving little space for roads and the growing number of cars owned by its 8.3 million inhabitants.
Pollution is a problem London is trying to solve--and its latest plan, Autocar reports, may involve a 22-mile underground ring-road to take some traffic off the city's streets.
It isn't simply an "out of sight, out of mind" problem. Road traffic will still be heavy, and London won't suddenly become pollution-free.
What the tunnel could do though is play to the automobile's strengths--making travel through London more efficient, while freeing up surface roads for pedestrians, cyclists, parks and buildings.
Immobility is a natural state for London traffic. Recent statistics suggest the average speed of a car during London's rush hours is little over 10 mph.
Journeys take far longer than they should, cars sit in long lines of stop-start traffic spewing fumes for no real benefit, and stress levels are high.
The traffic is also dangerous for cyclists--the city suffered a spate of cyclist road deaths earlier this year--and makes public transportation far less efficient than it could be, as buses and taxis also suffer in the traffic.
Most of those people just want to get from one side of London to the other, but the city's layout means there's no convenient way to do so. Drivers also pay heavily for the privilege--London's daily 'Congestion Charge' (actually a CO2-based emissions tax) is rising to almost $20 next month, from just under $17 per day.
The tunnel proposal, which runs underneath many of London's central boroughs, would feature as many as ten exits along its route, allowing passage to and from particularly populated areas--and free from lights and intersections, traffic would flow at a much greater rate than it does on the surface.
With large volumes of traffic now underground, surface space itself could be used for other things--new buildings, open spaces, or converted to cycle lanes.
London's iconic Tower Bridge, struggling under the weight of modern traffic (and a congestion hotspot due to its narrow width) could finally be closed to traffic, replaced by a nearby tunnel under the Thames.
It would be expensive: Current estimates put the work at over $50 billion, and projects like this aren't known for keeping to budget.
But it could also be vital. Transport for London suggests that congestion in the city could rise by 60 percent, by 2031.
Other cities have recently pursued similar paths--part of one of Madrid's urban motorways has been sent underground, and Hong Kong is working on a new tunnel too, replacing one of its many flyover routes.
There are, no doubt, plenty of issues to overcome, and London's plan is a long way from getting the green light. But could moving the bulk of city traffic underground be the best way of making our cities cleaner, quieter and less congested?