Share 'Something urban... something international... something on Earth... but what?'

Following the success of the mass exploration of London just over a week ago, I'm keen to find a theme and date for the next 'mass exploration'. Given that members of this network are all over the world I I reckon it would be awesome if we could get large numbers of us to go exploring during the same 24 hours and investigating/experiencing/sharing the same theme... The question is.. what, when a…