I read recently that more of The Underground is on the surface rather being actually underground!
That is true.
There are 272 stations in total, of which 156 are above ground.
When the original Underground lines were built in the late 18th and early 19th century, alot of the lines were totally under ground, withe the exceptions of the Metropolitan Railway and the District Railway.
With the expansion in the 1920's, '30's and 40's most were built above ground. The exception being the Victoria line built in the 60's which is totally underground except for the large depot in Tottenham.
I just checked, and approximately 23 miles are in cut and cover tunnels, ie the Met. and District.
And 93 miles are in the deep level tube system, Piccadilly, Northern etc.
So 106 miles out of a total of 250 route miles.