The comment is making fun of a proposal Boris Johnson once made about building a bridge from Scotland to Northern Ireland, which is considered borderline physically impossible as far as I understand.
Tunnel, no. It’s very feasible and practical from a civil-engineering perspective, but the economic value of a road connection between NI and Scotland has not been demonstrated to outweigh the economic cost of its construction.
I believe they have done similar things in Norway when they need to go across fjords which are about 1km deep on places.