Something about the idea of a "fixed" connection between Ireland and Britain sets so many people's teeth on edge; mostly, in Stormont and Holyrood, those of a nationalist persuasion.
Even though the recipient of an entrance point wouldn't be in the Republic, one can see how it might be a bit rich to have the "British connection", as it were, back up and running only a hundred years after independence.
That seems to be a powerful attitude, if a visceral and fairly illogical one.
Resistance to the idea of a tunnel mirrors the dismissals of the idea of a bridge - also a notion raised by the PM as he thinks about the changing relationships in these islands as a result of Brexit and renewed calls for independence referenda and border polls.
Whether or not any of those "separatist" initiatives come to anything, there is a clear need for the links, associations and relationships to be refreshed and reinvigorated.
There is something compelling about the idea of physical connections of that type; there is also a growing ambition among those who can make it happen, to do precisely that.
The tunnel proposals were submitted by the High-Speed Rail Group (HSRG), which comprises over 20 of the biggest companies involved in high-speed rail in the UK and the Railway Industry Association.
Contrary to the notion that the tunnel was pulled out of a hat to make unionists feel loved, the idea of physical links between these islands has been a theme of Boris Johnson's for some years now.
Setting aside the amusing comments from Tory MP and chair of the NI Select Committee, Simon Hoare, that "the trains could be pulled by an inexhaustible herd of unicorns overseen by stern, officious dodos", the arguments end up being about cost and priorities and the sense that the proposal has more than a touch of grandiose Victorian monumentalism about it, epitomised in the satiric tag "Boris's Burrow".
But none of these are actually very persuasive as to why it ought not be done.
No one has argued that trade would be more difficult, community relations damaged, free movement of people hampered, travel more expensive, parochialism promoted, jobs lost, tourism impeded, employment opportunities reduced, families split apart … In fact, all of these issues would be improved by an immediate physical link between the two islands. One could drive from Galway to Rome; you could live in Leeds and work in Belfast or Derry. One could work in Britain and be home in Ireland for tea. The drive is equivalent to Belfast to Downpatrick with only a fraction of the culture shock.
People might drift north on this island over time to be closer to the tunnel entrance. People might move here from England, attracted by a better lifestyle yet still easy access to loved ones home.
The tunnel at £10bn would cost a fraction of what the estimated £109bn HS2 railway link does - even cancelling that link would cost £2bn more than the cost of the tunnel.
Simply put, there's no financial argument against this revolutionary infrastructural innovation.
The Brexit breakthrough conversation in October 2019 between Johnson and Varadkar in the Wirral was a moment when the axis of communication shifted from London-Brussels to Dublin-London - and it hasn't really ever shifted back. There was a clear sense that a new context for Anglo-Irish relations could be in the offing.
Obviously, the controversies over the backstop first and now the Article 16 trigger tarnished those possibilities. But the EU sidelining Ireland in its unilateral deployment of the article only reinforced the sense that there are common interests at work in these north European islands which the EU doesn't really give a damn about.
The potential remains very attractive. The common language is a big draw for Irish people moving abroad; there are advantages for GB to have closer ties with Ireland as its nearest EU neighbour; a realignment of interests across the various nations as power and influence shifts would be to the benefit of all parties.
There is no diminution of sovereignty implied anywhere in the idea. Scotland, Wales, England, Ireland, Northern Ireland, would all clearly benefit. The construction jobs boost alone would be colossal.
There is more to be welcomed than feared. There is certainly nothing fantastical about any of it - yes, the Channel Tunnel was decades in the planning, but this is the 21st century now, not the mid-20th. Technology has advanced and ideology needs to shift with it.
In addition to the obvious material benefits of a connection by tunnel, there would also be a transformative process in attitudes, perspectives and relationships.
National-minded administrations and parties need to be cautious about resisting such change - the Scottish Parliament has refused to entertain the idea, though with no actual reason why. Scotland enjoys full cross-border access to the riches of the big English economy. So does Wales. Why it would be keen to deny such access to Northern Ireland or Ireland is a mystery, especially amid endless talk of 'Celtic solidarity'.
Such a tunnel would help to reassure northern unionists while bringing substantial benefits to the island as a whole. It has no constitutional implications and does not have anything to say about a border, but it does have a lot to say about trade, movement and communication.
Let's face it, if Westminster is planning to raise the money to build it, why on earth would anybody say no?