There was an interesting article in yesterday's Times about steps taken to protect the water ways approaching central London.
There was concerns that the Thames could be used as a soft underbelly to the capital, and questions about whether it is sufficiently patrolled.
Well it already seems pretty well patrolled: you can't sit and watch the river for long before seeing one of the Port of London or Police boats. But apparently that's not enough and the Special Boat Service (SBS) is on stand-by.
It sounds dramatic, but lets get a sense of proportion. See the picture above? Its a pill-box, one of 28,000 odd built to defend Britain against possible invasion in World War 2.
This one is just by Putney Bridge tube station, just on the north side of the Thames.
So think for a moment what it was designed for. If Britain had been invaded by the Nazis, and they had successfully landed, broken out of their breach head, and had reached London, then the Thames would be a defensive line.
And this pill-box would be the front line, trying to save what was left of Britain, as the battle raged through the streets of London and the bridges are blown to stop further advances.
Remember we didn't know we would win - we could have lost. The army might have been trapped at Dunkirk and the little boats unable to rescue them.
London survived the Blitz, and it survived the IRA bombs: its a resilient city.