A bridge from Ireland to Britain is among the ideas currently under review by engineers in both countries.
While bridges are normally cheaper than tunnels, any proposed bridge between the UK and Ireland would have to be incredibly long. The longest sea bridge in the world at 20.2 miles long is the Donghai bridge that links Shanghai to Yangshan in China.
A bridge from either Northern Ireland or the Republic of Ireland would have to be even longer than that.
In 2007, a 21-mile rail bridge was proposed between Galloway, Scotland, to Belfast, Northern Ireland. The idea would have trains running from Dublin, through Belfast, over the sea to Scotland, and then down railways into London.
The idea hasn't been acted upon, but the concept still stands. Recent proposals suggest four different options for passage over, or under, the sea, reports the BBC.
"Four main routes have been proposed. Two run from Scotland to Northern Ireland - Campbeltown to County Antrim, or Stranraer to Belfast. Two run from Wales to the Republic, a northern and southern route, where the Welsh peninsula juts into the Irish Sea."
The northern route to Wales would stretch from Dublin to Holyhead, while the southern route extends from Rosslare to Fishguard.
Bill Grose, the former chairman of the British Tunneling Society, says that there are "two critical issues for siting the tunnel."
The first would be finding a location that maximizes the demand for transport across the sea and how well the location is served by existing infrastructure at either end of the connection.
The second would be the shortest distance across the sea. Between Dublin and Holyhead is roughly 50 miles of water, Waterford and Fishguard is 45, and Belfast to Stranraer is around 20 miles.
The route from Antrim to Campbeltown covers only 12 miles of sea, but because Campbeltown is in an isolated part of the country with little existing infrastructure, transport links would need to be established to cut through some mountainous terrain.
"Intuitively Holyhead to Dublin is a more preferable route than the others. It's closer to Manchester and Liverpool and connects straight into Dublin," says Grose.
A rail tunnel or bridge would be a more realistic goal to accomplish, from both an engineering and financial standpoint. Rail tunnels cost around $93m per kilometer (70m euros), while road tunnels require more space as well as a ventilation island halfway across.
The endeavor to establish a connection between the nations would be an expensive one no matter which way you slice it. Also considering that most of the benefit from the bridging of the countries would be to Ireland, the Irish government would be expected to put up at least half of the cost, something that seems unlikely given its current economic state.
Happy Earth Day! Ireland sure looks gorgeous from outer space