TARIFA, Spain – Engineers have dreamt of it for a quarter-century: linking Europe and Africa at the spot where the two very different worlds gaze at each other across a strip of choppy water.

Now, after seemingly endless studies that turned up more than one nasty geological surprise, a project for a high-speed rail tunnel connecting the continents is gathering momentum, raising the prospect of an engineering marvel on par with the Panama Canal or the Channel Tunnel between Britain and France.

This tube for passengers, cars and freight would bore deep under the Strait of Gibraltar, the narrow waterway where the Atlantic flows into the Mediterranean, and run from Tangier, Morocco, to the Spanish town of Tarifa at Europe’s southernmost tip, possibly extending further both ways in the future. Big-name European engineering consultants brought in a few months ago are to complete a feasibility study this year.

“I think this project is a utopia that is becoming a reality,” said Angel Aparicio, president of the Spanish government agency overseeing the endeavor with Moroccan partners.

Aside from fueling economies on both sides of the strait, planners are excited by the symbolically powerful feat of bridging two continents as far apart socially and culturally as they are close geographically.

But the technological challenges are mammoth: A test tunnel dug outside Tarifa a decade ago, for instance, unearthed a smorgasbord of soils, some on the mushy side, hardly the right stuff for anchoring such a grand structure.

The cost is unofficially projected to run over $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.

There’s also the issue of whether that economic chasm between Europe and Africa would doom the tunnel as a white elephant. Planners wonder whether Africa is too poor to provide a sustained, profitable flow of people and goods on the northbound leg of a tunnel.

Even the popular Channel Tunnel opened in 1994 has accrued $16 billion in debt and the company operating it, Eurotunnel, received bankruptcy protection from creditors last year. The costs of digging the 30-mile undersea rail tunnel were massively underestimated, and traffic predictions proved optimistic.

Still, Spanish Prime Minister Jose Luis Rodriguez Zapatero said last month he is fully committed to the Strait of Gibraltar project. He said the tunnel would “greatly speed growth, development and prosperity” on both sides of the Mediterranean.

Planners hope the tunnel will create “an integrated Euro-Mediterranean economic area” and be more than just a way to cross the strait, a journey now done by ferry.

They envision a day when tracks from the tunnel would stretch as far south in Morocco as Marrakech and allow for travel time just a fraction of what it is now.

Morocco’s tourism industry, which the government has marked as a key economic motor for the future, would benefit from a tunnel.

“Tourist flows will accelerate because people will be able to come with their own transport. For now, the need to cross by boat presents a psychological and practical barrier,” said Tajeddine el-Husseini, professor of international economic law at Mohamed V University in Rabat.

The biggest winners in Morocco would probably be exporters, who could ship their goods north more easily, mainly agricultural products. Being able to send them to Europe by train rather than ship will also make for easier export of fragile products like tomatoes and flowers.

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 nine miles 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 22 miles. Even there, however, the water is about 1,000 feet deep, five to six times deeper than the water in the English Channel where the chunnel runs.

Then there is the messy terrain at the bottom of the Strait. “It is chaotic. The word is chaotic,” said Sebastian Sanchez, an engineer overseeing the tunnel test site in Tarifa.

It is muddy and unstable right at the seabed, unlike the harder surface at the bottom of the English Channel, then farther down are huge pockets of debris from tectonic slides – a cocktail of sand, stones and mud that make for a digger’s nightmare.

The proposed two-tube link with a service tunnel in the middle would be about 1,650 feet below the surface of the water, the deepest underwater tunnel anywhere.

To these two challenges – extreme depth and uncooperative terrain – add the concern over whether the project would be economically viable. One key study under way aims to determine if Africa’s poverty would make the tunnel busy in one direction but largely idle in the other.

“Nowhere else in the world do these three difficulties come together,” Aparicio said.

Aparicio’s agency and the Moroccan partner will recommend in 2008 whether to go ahead; it is not clear when the governments would make a decision.

In Tarifa, a windy town of 10,000 known as a world magnet for kite surfing, many say the tunnel idea has been around forever and will remain a distant dream.

“People here don’t see the project as something tangible,” said Mayor Miguel Manella. “They say ‘I’ll never live to see that.'”