https://en.wikipedia.org/wiki/River_Fleet
Looks lovely from the old paintings, there’s supposed to be barge moorings in what is now the sewer.
The Thames overflow comes out under Blackfriars bridge, however the the Thames Tideway tunnel is being built to reduce overflow events:
https://en.wikipedia.org/wiki/Thames_Tideway_Scheme
So there’s the Crossrail tunnel, Thames Tideway tunnel, and the recently completed National Grid tunnel to carry extra electricity through London.
https://en.wikipedia.org/wiki/London_Power_Tunnels
