The Channel Tunnel linking Britain and France holds the record for the longest undersea tunnel in the world – 50km (31 miles) long. More than 20 years after its opening, it carries more than 10 million passengers a year – and more than 1.6 million lorries – via its rail-based shuttle service.

What many people don’t know, however, is that when owner Eurotunnel won the contract to build its undersea connection, the firm was obliged to come up with plans for a second Channel Tunnel… by the year 2000. Although those plans were published the same year, the tunnel still has not gone ahead.

The second ‘Chunnel’ isn’t the only underwater tunnel to remain a possibility. For centuries, there have been discussions about other potential tunnelling projects around the British Isles, too. These include a link between the island of Orkney and the Scottish mainland, a tunnel between the Republic of Ireland and Wales and one between Northern Ireland and Scotland.

You might also like:

• The hidden cost of transport ‘megaprojects’

• The controversial plan to tunnel beneath Stonehenge

• The monster tunnelling beneath London’s streets

Some of these tunnels yet may happen: even the Channel Tunnel built for the railway in the 1980s was the culmination of nearly 200 years of thought and discussion.

At the time, British Prime Minister Margaret Thatcher’s own preference was for a road tunnel, not the current railway service. She liked the idea, some believe, because cars “represented freedom and individualism”.