The cost is unofficially projected to run over US$13 billion and engineers say the tunnel would take about 20 years to construct. Spain and Morocco hope to receive European Union financing if the project gets under way.
For decades the Strait of Gibraltar was a dangerous and often deadly conduit for Africans seeking to reach Europe, as people packed small, rickety boats to try to reach Spain and gain a toehold on the wealthy continent. Because of a Moroccan security crackdown, these journeys are now attempted further west from the western Sahara and Mauritania.
Ferries that sail across the strait are not known to be a major lure for stowaways, so an extremely high security rail tunnel would probably not contribute either to the flow of desperate Africans trying to reach Europe.
The Strait of Gibraltar, formed millions of years ago when land masses split to form what are now Europe and Africa, is only 14.5 kilometres wide at its narrowest point. But the water is so deep there a rail tunnel would be like a roller coaster slope, so steep as to be out of the question.
So engineers have chosen a longer but shallower path spanning about 35.5 kilometres. Even there, however, the water is about 305 metres deep, five to six times deeper than the water in the English Channel where the chunnel runs.