Researchers at Heriot-Watt University in Edinburgh are exploring the possibility of a "submerged floating tube bridge" between Northern Ireland and Scotland\ writes Irish Central quoting Times.

This submerged tube would be floating 50m below the sea surface. Vehicles would not directly run in it but instead would be transported on shuttle trains cutting the 3-hour current ferry journey to some 50 minutes.

https://www.irishcentral.com/news/thenorth/tunnel-northern-ireland-scotland