TWENTY years ago, the Channel Tunnel was marking its first week of operation with trains carrying passengers 23 miles under the sea bed to France and back.

It's easy to forget what an incredible engineering feat this was.

The tunnel runs for 31.4 miles, with 23.5 miles under the sea, giving it the longest undersea section of any tunnel in the world.

The project was also an impressive example of co-operation between Britain and France.

Although from opposite ends of the political spectrum, Margaret Thatcher and Francois Mitterand ensured their countries worked together, treaties were signed, a common company was formed and the work finally began.

This co-operation was apparent when I travelled from Folkestone in 2010 to France.

French border guards were stationed at the British end and vice-versa.

Signage and styling was a perfect blend of British and French.

It was an example of how nations can work together given the vision and the finance.

Although I was enormously excited about this achievement, I didn't actually go on it until 2010.

Many seemed to share this reluctance to use the tunnel, impressive though it was.

Ferry companies slashed prices to compete and more importantly, the tunnel's opening co-incided with the launch of low-cost airlines.

All this has meant that the tunnel has never carried the number of passengers it was expected to do, and has never made the money its backers wanted.

It has been beset by other problems, notably closure due to fire in 1997 and illegal immigrants hitching rides on trains from a refugee camp near Calais.

Has it been worth it though? Undoubtedly it has. Passenger volumes, while not as big as tunnel bosses would have liked, have been growing. For the jaw-dropping scale of the achievement, one that makes what was a difficult journey, very ordinary - it has been worth it.

Once engineering feats like this - vast tunnels and stylish bridges seemed to be the preserve of other places; Austria, the US and Japan. But I remember sitting in my car as it sped under the sea at 100mph, having had my check-in speeded by the automatic number plate recognition; having quickly gone through passport control and boarded the train; being so impressed and so very proud that this very slick operation was, in part, a British achievement. And that’s what makes it worth it.