A look at the VM (and other) underground cable tunnels in London.
 

I think this has been made more secure now to help stop terrorists from disrupting the backbone of the internet etc.

It looks like VM, BT etc colour code their cables:

https://m.youtube.com/watch?v=jRqT3932rkU&t=2407s

I had to laugh at one of the comments from someone who said "no wonder my internet is so bad" after seeing the jumble of cables right at the end :D