This is the floodplain of the River Thames in London.
It has not only been built over, the river is now much narrower than it was because land has been reclaimed on both banks.
Great docks were once dug into the muds of the floodplain. Later, when the docks were no longer needed, the docklands area was made into offices, housing and waterside leisure. That is what you see here...
Playing time: 5 minutes.
