In December Prime Minister Boris Johnson said he was thinking about building a bridge across the sea linking Scotland to Northern Ireland.
The PM said a road between the two countries, joining the UK together, was a "very interesting idea" and to "watch this space".
Now the Institution of Civil Engineers (ICE) says it has come up with an even better plan.
The engineers want to dig two high-speed rail tunnels under the Irish Sea to link the four countries of the UK together for the first time.
They say that, unlike a bridge, tunnels won't be affected by bad weather.
The two tunnels could run from England to the Isle of Man, and from the Isle of Man to County Down in Northern Ireland.
The distance from England to the Isle of Man is around 50 miles, and from the Isle of Man to Northern Ireland is just over 30 miles, but it's not yet clear exactly where the tunnels will start and finish.
Advances in technology means tunnels are faster and cheaper to build than ever.
The engineers estimate the cost of the tunnels could be around £15 billion - a similar price to a bridge.