The UK government is investigating the possibility of digging a tunnel between Scotland and Northern Ireland, the Scottish secretary has said.
Prime Minister Boris Johnson has previously raised the idea of a bridge being built across the Irish Sea.
But Alister Jack told MSPs this was a "euphemism" for a tunnel, saying "it would be less expensive to tunnel it".
Nicola Sturgeon had distanced herself from the bridge plan, saying there were "more important priorities" for funds.
The Scottish Greens said the idea was "pure fantasy, just like the Tory approach to Brexit".
Mr Johnson has previously described the idea of a bridge between Scotland and Northern Ireland as being "very interesting", with two different routes being considered by officials.
However in an evidence session with Holyrood's Europe committee, Mr Jack said that "the bridge for me is a euphemism for a link which is a tunnel".
The MP said: "It's no different to the tunnels connecting the Faroe Islands, it's no different to the tunnels going under the fjords.
"Tunnelling techniques now are quite advanced and certainly to go from south west Scotland to Northern Ireland, it would be less expensive to tunnel it."
Asked by SNP MSP Kenny Gibson if there could be issues with "people being asphyxiated" in such a tunnel, Mr Jack said the route would be "the same length as the Channel tunnel, and we're not asphyxiating them as best I know at the moment in the Channel tunnel".
He added that there are "much longer tunnels in China", and that a tunnel would deal with the issue of Beaufort's Dyke - a dump of World War Two munitions - as well as "the problem of weather", estimating a bridge might have to be closed for 100 days a year.
Mr Jack added: "The tunnel is at discussion stage. It may move to feasibility stage. In the interim we can start to have the debates about whether it's viable, whether it's going to help the Northern Ireland economy and the Scottish economy.
"Once we get better sight of the costs involved, should the prime minister decide to press the button, we would then want to engage with both Stormont and Holyrood to get a better understanding of the benefits and and the challenges.
"We're not going to just come riding roughshod and slam a tunnel in - and by the way under the settlement of devolution, nor can we."
Ms Sturgeon had previously said her mind was not closed to the idea of a bridge, but said that if Mr Johnson "has got £20bn to build such a bridge going spare at the moment, that could be spent on more important priorities".
Green MSP Ross Greer, who is on the committee which was questioning Mr Jack, said the latest plan was "pure fantasy".