How Much of the Tube is Actually Underground?

It's the sort of thing that turns up in quizzes and that I get asked a lot ... how much of the London Underground is actually underground? Turns out, there's not a straightforward answer as it depends on how you define a station as being 'Underground' ... let's discuss! Geoff's Actual Underground Map: http://www.geofftech.co.uk/downloads/... TfL's Tube Map Showing Tunnels: http://content.tfl.gov.uk/tube-map-wi... The Section 12 Fire Regulations Map: https://www.whatdotheyknow.com/reques...