Channel Tunnel operator Eurotunnel tomorrow celebrates the 20th anniversary of the opening of the undersea link between Britain and Continental Europe.
A reception and an exhibition on the history of the Folkestone to Coquelles link will be held at Eurotunnel's terminal in northern France.
Formed of twin railway tunnels and a parallel service tunnel 100 metres below sea level, it is widely considered to be the 20th Century's most important construction project.
The 31-mile tunnel has become a key part of Britain's infrastructure after the Queen and France's then president Francois Mitterrand inaugurated it on May 6, 1994.
Its opening marked the culmination of decades of Anglo-French talks on the idea of a fixed link between England and France.
A cross-Channel fixed link was first proposed in 1802 by French mining engineer Albert Mathieu, using oil lamps and horse-drawn stagecoaches.
The following year an Englishman - Henry Mottray - unveiled another cross-Channel link project - a submerged tunnel made of prefabricated iron sections.
For decades, the idea of a cross-Channel fixed link had been discussed, only for the plans to be discarded on security grounds.
But years of peace in Europe following the end of the Second World War finally encouraged UK and French leaders to go ahead with the project.
In July 1957, a Channel Tunnel Study Group was formed and three years later the group presented to the governments plans for a twin rail tunnel with a service tunnel.
Some 13,000 engineers, technicians and workers helped construct the tunnel, and 11 boring machines each weighing 1,100 tonnes were used in the excavation.
Geoff French, president of the Institution of Civil Engineers, said: "The Channel Tunnel is a true feat of engineering.
"It contains one of the longest segments of undersea tunnel in the world, established the first fixed link from England to Europe, and over 13,000 workers from England and France collaborated to realise the vision."
Operations leader Christine Marsh, 60, from Dover, has worked at the tunnel from its opening - and recalled its early beginnings.
She said: "It was very quiet back then, everything was so new and exciting. We had to do a lot of training to face the challenges.
"I was here when the Queen visited. We all knew it was going to be a very big thing and that we were part of something new."
From the start, the tunnel was dogged by controversy. Ten years after it opened, Eurotunnel had debts of £6.4 billion and original traffic forecasts proved to be wildly optimistic.
Today the picture is markedly different. Last year Eurotunnel achieved revenues of one billion euro for the first time.
And last month Eurotunnel posted increased revenues of about £217 million for January-March this year - up 8% on the same period last year.
But it still faces stiff competition from the popularity of cross-Channel ferry services and from low-cost airlines.
Eurotunnel's commercial director Jo Willacy said: "Commercially, it has been a tremendous success. One in two vehicles that now crosses the Channel, crosses with us.
"Not only that, but the market is bigger now as a direct result of the Channel Tunnel being brought to the market. It's now hard to believe it didn't exist.
"We are the leading choice for crossing the Channel and we can do so due to introducing a service that delivers speed and is simple.
"No longer is crossing the Channel a block or a stopping point. It's a smooth experience. There are always challenges for any business, but we are a mature business.
"We are now part of the national infrastructure for the UK and the Continent. I don't think there is anything that we can't do."
Although officially opened on May 6, 1994, the tunnel did not open to freight traffic until the following month and it was not open to passenger services until December 1994.
A devastating fire in the tunnel in 1996, which led to the suspension of freight services for many months, was a big blow to Eurotunnel, as was the problem of asylum seekers boarding trains.
Eurotunnel's John Keefe said the tunnel has helped revive a region as a centre for rail and infrastructure.
He said: "We had up to 15,000 people employed here during construction, and we have got 830 people here on the site directly, with another 3,000 contractors working for us.
"If you go beyond that and look at Cheriton Park, Orbital Park, these are all places that were part of the development around the Channel Tunnel.
"Then you have got the M20 motorway which was brought down to link up and HS1.
"Just from the small seed of an idea to build a Channel Tunnel, all of a sudden Kent has expanded enormously."