UNTIL this week, a 'levee' had been a meaningless word that Don McLean had invented to fill up the chorus of his long-winded 1971 hit:

So bye bye, Miss American Pie,

Drove my Chevy to the levee

But the levee was dry.

Now, though, the world knows that the levee has been overwhelmed, the city behind it is flooded and descending, through anarchy, into chaos.

How New Orleans came to find itself sandwiched between two levees is an accident only history can explain. To the north of the city is the levee holding back Lake Pontchartrain; to the south is the levee restraining the Mississippi River as, at the end of its 2,348 mile journey, "the father of waters" pours out through marshes into the Gulf of Mexico.

In the hollow in between is the city founded by a Scot on behalf of the French in 1718.

This part of America had been discovered by the Spanish in 1541 but claimed by the French in 1682. They couldn't make it work, though, and so Philippe, Duc d'Orleans, granted all the commercial rights to his friend, John Law. Law was a splendid fellow: a drinker, a gambler, a chancer, a dandy. He'd been sentenced to death in England for murdering his opponent in a sword duel. By drugging his jailers and filing down his fetters, he'd escaped to France where he took up with Philippe. They set up the first national bank of France and when Philippe became Regent in 1715, he gave Law the right to exploit Louisiana.

A port was needed to give access to the Gulf and to the river which would carry the settlers inland. Law named this port after le Duc, and shares in the Mississippi Company soared from 500 livres to 15,000 livres, such was the new confidence.

Then investors realised that there weren't the untold mineral riches in Louisiana that Law had promised. The shares plunged, the national bank collapsed, Law went bankrupt big time.

Nouvelle Orleans struggled. For starters, there were too few women to populate the place, and more significantly, Orleans was proving extremely difficult to build. It was hot, humid and bedevilled by mosquitoes and plague, plus the waters of the Mississippi fluctuated wildly. To keep the river at bay, raised roadways were built - 'levee' comes from the French verb 'lever' 'to raise' - and the houses and plantations huddled behind them.

Ever since, the levees have been battling to keep the water out. In 1803, Napoleon sold the 600 million acres of Louisiana to the US government for $15m - the biggest land deal ever - because thousands of his soldiers had died of yellow fever, contracted from mosquitoes in the Orleans swamps.

It formally became the 18th US state, but was known as "a damp grave". Spring floods washed in two feet of water, debris, mud, snakes, and were usually followed by epidemics.

And so the levees went higher and higher and longer and longer...

But, in protecting the city, the 2,000 miles of levees were also contributing to its downfall. They enabled the city to dry out, causing it to sink. They re-directed the Mississippi so that it no longer deposited its mud on the wetlands, allowing more houses to be built on hitherto uninhabitable swamps.

But the wetlands were Orleans' last line of defence before the levees, acting as a natural buffer to strong winds. Without them, sinking Orleans only had its levees. In places they were 18ft high - but Katrina's storm surge produced 30ft of water.

The levee was no longer dry, and the Chevy was washed away.