posted earlier was from the Thames Tunnel, once described as the 8th wonder of the world.
It was the first tunnel built under a navigable river, constructed way back between 1825 and 1843 by the 20 year old Isambard Kingdom Brunel and his dad, Sir Marc.
It almost killed him. There were several floods and half a dozen workers were downed, and when we went through there was an ominous sound of water trickling from somewhere:
So after it finally got built it was eventually taken over by the world's oldest underground railway, the tube.
It took about fifteen minutes to walk to north London and another fifteen minutes back again, passing this plaque:
And the reason for all that effort, the piles of money and the death of the workers?
The docks, filled with thousands upon thousands of ships, whose tall masts prevented a bridge being built but drove demand for better transport links between south and north of the river.
Even deep underground you can't escape London's maritime history.
Update 1: there's a BBC video from inside the tunnel here.
Update 2: the ITV news article on the tunnel walk seems to have difficulty with the concept of 18th vs. 19th century.