Ah, that's because there's actually a small slot up to daylight.
If you stand on the westbound Central platform and look up you can see.

I only realised because I managed to get rained on there once
( Gizmo.mp3 does not exist, Fri 1 Sep 2006, 12:52, archived)

That was just a tramp pissing on your wafro

( Amorous Badger NAKED BEA ARTHUR PHOTOS 4U, Fri 1 Sep 2006, 12:56, archived)

i know
and you missed most of the east and west ends of the District line
( Clairebare Back once again with the Ill behaviour, Fri 1 Sep 2006, 13:00, archived)

Rosie had already mentioned those.
What I entirely missed out was the huge stretch of Metropolitan line out to Amersham/Watford that's entirely open...
( Gizmo.mp3 does not exist, Fri 1 Sep 2006, 13:20, archived)