Just before getting out of the tunnel arriving at the station southbound, the wall of the tunnel begins to curve away, for just far enough to see it's doing it, and then there's a barrier built across the hole, nearly parallel to the running line, made out of what looks like wood that may have been painted some colour other than black once upon a time underneath all the dirt. Not much to see and I had to go past it a couple of times to put the picture together.
However I have seen a cab ride video made after I was last there (which was quite a while ago) and it looks like they've Done Things in the meantime and it might not look like that any more. It's hard to tell anything because the quality isn't great and the camera can't handle the contrast between light and dark, as well as it not looking sideways, but it does look rather as if they might have hidden it.
See above conundrum re Chalton St Junction. The Midland built a spare extra bit of tunnel when they built St Pancras just in case it was ever needed for anything, to save the chance of having to have the forecourt dug up all over again. The eastern end of that tunnel now has the eastbound platform of the new Met/Circle station in it. For a few years before the new station was built, the tunnel was used for a loop line connecting the eastbound Met to the WL (the track being laid on the southern side of the tunnel, where the platform now is) and at this time the tunnel may have been partly extended, for one track only, no more than half way towards Euston; a contract was awarded to excavate this by cut and cover, but I haven't been able to dig anything up that would confirm to my satisfaction how much if any of it was actually executed. There is at least some extra length remaining unused down there, down the mysterious hole off the western end of the eastbound Met platform, which must go at least as far as the western edge of St Pancras forecourt, but I can't be sure whether there is or ever was any extra little wormhole running off the end of that. If such was ever built it was never used or intended for anything other than the bit of loop line.
Everyone these days says "to Euston" when talking about this, but that was never the idea. The LNWR were never interested in having a connection to the Met, and declined when asked. They had the North London route to Broad St, which gave them a better connection to that end of things than could ever be achieved over the congested and cramped Met route.
When the Victorians were kicking the idea around, it was "to Paddington" that they used to say. The GWR did want a better connection to their City goods terminal than was possible over the Met as it stood. This was still the idea after WW1, which led to installing the abovementioned loop line (and the associated crossover for the opposite direction) - it would allow GWR freights the most advantage possible from the Widened Lines in their existing form, without going to the hassle of building a proper extension.
