I was standing half way along the southbound Victoria Line platform at
Kings Cross, when I happened to look up through the vents in the
roof. Through them, I could see daylight, coming in through what
appears to be another tunnel running more or less parallel to the
Victoria Line.

I've used this platform many times, but have never noticed this other
tunnel before. Is it new (in some way connected with the northern
ticket hall works), or has it always been there? If so, has something
been done to allow light into this tunnel?

Or, has it always been like this, and I've never noticed.

Cheers,

Simon