There's a distinct and unique smell to the London Underground.
Not the sweaty scent of cramped carriages but the whiff of earthy engines wafted through the platform by approaching tube trains clattering towards the station, or overflowing from below when glancing past a tube entrance.
What is it that makes London smell like London? Or any city for that matter?
While everyone cares about the look, layout and landmarks of a city, there's one group of researchers who think the urban smellscape is just as important.
There's a heady aroma of car exhaust fumes, fuel and dust overpowering Londoners' nostrils (perhaps unsurprisingly). But not far behind, the smell of the natural world - flowers, plants, trees and grass - is enveloping our noses.
Meanwhile in Barcelona, the scent of food in the city is the strongest.
Researchers from Cambridge, the Royal College of Art, Yahoo Labs and the University of Torino, have managed to track the distinct aromas of a city through "smellwalks" (literally taking people on walks of the city and getting them to write down what they smell) to create the first urban smell dictionary.
Here's the urban smell dictionary, turned into the urban smellscape aroma wheel.
Social media updates from Twitter, Instagram and Flickr, which were tagged with locations were then matched with the smell dictionary to come up with the smell maps.
Here's London's emissions smells.
"At street level, streets with emissions words (e.g., Kensington Road and Park Lane in London) suffer from air pollution, while streets with nature words (e.g., Hyde Park) do not. Where do you expect to find animal smells? In the zoo, of course," said the researchers - Daniele Quercia, Luca Maria Aiello, Rossano Schifanella, and Kate McLean.
Here's what London's animal smells look like mapped.