Officials in the former communist Czechoslovakia had planned to build an underground rail link under Austria to the Adriatic, a Czech newspaper says.
The 410km-long (255 miles) tunnel would have drastically shortened the journey to the seaside for the landlocked country, Lidove Noviny reports.
The earth from the excavation would be dumped in the sea to form an artificial Czechoslovak island - Adriaport.
But the 1975 plan never got off - or under - the ground, the newspaper says.
The plan was proposed by Professor Karel Zlabek.
He devised a railway tunnel that would have linked the town of Ceske Budejovice, in south Bohemia, and the port of Koper in what is now Slovenia, then Yugoslavia.
The project envisaged that high-speed trains would take both tourists and exports to the Adriatic coast in about two hours.
Work never started on the tunnel.
But that was not because it was totally unrealistic, the BBC's Rob Cameron in Prague says.
Modern-day engineers say building work could start tomorrow.
But the project failed to address several key questions.
Apart from the sheer expense, one of the key issues was whether the Austrians would have let a Communist country build a tunnel underneath their feet, our correspondent says.
He adds for decades many Czechs have headed each summer to the Croatian coast in search of sun, sea and sandy beaches.