The first London Bridge was constructed by the Romans around the year 60 AD. Since then there have been at least 5 versions of the bridge.

London owes its existence to a ford that apparently still exists beneath the Thames mud opposite the Houses of Parliament.

Julius Caesar's legions took this route before going on to storm the heights to the north, such as Primrose Hill, Hampstead, and Highgate. They might have followed the line of the River Fleet, which now flows through the sewers beneath the street that bears its name.

As well as being the lowest fording point, London was the highest reach of the Thames where a sea-going vessel could unload its cargo. It could also be conveniently bridged, which the Romans achieved around 60 AD.

Since Roman times London has fallen by 15 feet, a reaction to the end of the ice age 25,000 years ago that weighed down much of Britain under ice. Northern Britain is 'springing back up' after being covered in ice, whereas southern England is sinking to counter act the north. So whereas in Roman times London Bridge marked the end of the sea's tidal influence on the Thames, today that point reaches a further 19 miles upstream.

In 850 Danes destroyed the first bridge. The rebuilt version collapsed due to heavy floods in 1091. The replacement for that burnt down in 1136.

The first stone version, partly paid for by a wool tax, was started 40 years later. This bridge partly collapsed in 1281, but when restored, this time with houses, shops and a chapel on it, stood until 1831.

The next version remained until 1971, when it was sold to become a tourist attraction at Lake Havasu City, Arizona.

The old nursery rhyme that said 'London Bridge is falling down' is quite appropriate.