Spain and Morocco say they have agreed to build a 39-kilometre rail tunnel beneath the Mediterranean Sea, to link Europe and Africa.
The Spanish Development Ministry said two adjacent tunnels will run beneath the Straits of Gibraltar to Morocco.
They are planned to run between Punta Palomas, 40 km west of Gibraltar and Punta Malabata near Tangiers.
The ministry said the route had been chosen because the Mediterranean at that point is just 300 metres deep.
The ministry said that a joint Spanish and Moroccan committee of officials had already agreed a preliminary three-year plan of works to start as early as next year, with an estimated initial cost of $30m.
The vision of a fixed link between Spain and Morocco has been alive for more than 20 years, and was revived at a summit in Morocco earlier this month.
Spanish-Moroccan relations hit a low in July last year
It comes a year and a half after a crisis between the two countries over the tiny island of Perejil.
Spanish marines forcibly evicted some Moroccan soldiers from the island, which both countries claim.
The two countries are also at odds over Western Sahara, where Morocco suspects Spain of favouring the Polisario Front independence movement, and the Spanish North African enclaves of Ceuta and Melilla, which Rabat says are occupied territory.
This plan could be put into action as early as next year but the eventual total cost of the project is likely to be much more.
The proposal has drawn comparisons with the Channel Tunnel, that links England and France, and raises the prospect of a continuous rail link between the north of Scotland and Africa.