Will a tunnel from Spain to Africa ever be built – and who would use it?

It is one of those concepts that has always seemed theoretically possible, so short is the distance between A and B - and yet, equally, like a flight of fantasy from a Jules Verne novel, unrealistic beyond the realms of imagination.

It floats between the two extremes - wholly conceivable on the drawing board, and yet, also, the engineering equivalent of a dragon or a unicorn in the simple facts of nuts and bolts. So near, but also, so far.

What is this referring to? To the cluster of cranes on the horizon that has long been mooted, but never attempted - a tunnel that runs underneath the Strait of Gibraltar. As Verne himself put it in his sub-aqua spectacular Twenty Thousand Leagues Under The Sea, "it was obvious that the matter had to be settled, and evasions were distasteful to me."

So it has proved this week, with fresh murmurings as to how, why and when such a blast from the future could be created. Remarks, initially reported in the Andalusian newspaper Diaro de Cadiz - attributed to the Spanish planning group SECEGSA, and its president Rafael García-Monge Fernández - have suggested that not only is the idea of a "land" link between Spain and Morocco feasible, it is a proposal where wheels are beginning to turn. To the point that the number of drills needed to carve out the thing, and the cost of making them, has already been assessed.

Once you have asked yourself what "SECEGSA" stands for, discovered that it means the "Sociedad Española de estudios para la Comunicación fija a través del Estrecho de Gibraltar, S.A", and wondered whether this name - the "Spanish Society of studies for fixed Communication through the Strait of Gibraltar, S.A" - is longer than any tunnel under said body of water could ever be, you will probably come to the conclusion that, a) yes, such a labour is possible, and b) there are precedents. SECEGSA's research has figured that the length of this construction would be 24 miles (38.67km) in total, with 17.2 miles (27.75km) cocooned below the seabed.

If these numbers sound plausible, it is because they are. The Channel Tunnel stretches out for 31.35 miles (50.45km), with 23.5 miles (37.8km) of its structure submerged - while the Seikan Tunnel, which connects the Japanese islands of Honshu and Hokkaido, extends to 33.46 miles (53.85km), with 14.5 miles (23.3km) of its torso ebbing below the Tsugaru Strait. It is not for pie-in-the-sky engineering reasons that a tunnel between Europe and Africa does not yet exist.

So why hasn't it happened? The first and obvious matter is one of finance. Who would pay for it? SECEGSA says that Spain and Morocco are both interested in pursuing the project - and seeing as SECEGSA is a Spanish government body (its website is secegsa.gob.es), we can presumably trust its word on the former.

But its latest musings, and its dismissal of an earlier estimate of an overall cost of €8billion as utterly wide of the mark ("that figure has nothing to do with reality," Fernández said this week), mean that broader funding from the European Union, as well as private finance, will be essential. And that will lead the tunnel down a twisting road of bureaucracy and corporate tin-rattling from which it might never feasibly return. The price of producing a prototype version of one of the eight drills necessary to cut this long path under the narrowest portion of the Mediterranean is, alone, put at €32million.

Then there are the physical issues. While the tunnel would be shorter than its siblings in Japan, and between Folkestone and the Pas-de-Calais, the state of the seabed between Spain and Morocco is a cause for concern. Surveys of the shortest line of construction have identified two distinct zones of about 2.5 miles (4km) in length where the predominant soil, difficult to build through, is soft clay. SECEGSA, supported by researchers at the University of Zurich and the German tunnel-boring company Herrenknecht, says that this is not insurmountable - but it will make for another likely delay in a tricky process.

There are other complications. At its current proposed length, the angle of the tunnel as it emerges onto each landmass would be around three per cent - too steep for the high-speed rail line that would be the main point of constructing it. To achieve a flatter curve, and the angle of 1.2 per cent which is generally considered to be the maximum for technologically advanced train services, the tunnel would need to be rather longer.

Finally, there is a blunt but not entirely unreasonable question. Why bother? The Strait of Gibraltar is already ably served by ferries and boats chugging from Algeciras (in Spain) to Tangier Med (in Morocco). Indeed, the Moroccan government has pumped enormous sums into Tangier Med, a port which now has a cargo capacity of eight million containers. Would an underwater train link complement this growing harbour's efforts, take work away from it - or be rendered a white elephant by its prior existence?

For all this, however, there is something eternally evocative about a tangible link between continents - an idea which conjures dreams of long journeys to distant places.

Not least because such connections scarcely exist. Beyond the Bosphorus Bridge, which soars between the European and Asian sides of Istanbul, there are no clear locations where you can make such a leap. Cartographers can argue as much as they wish about the precise junction boxes which glue Asia to Africa and South America to its Northern counterpart, but political and geographical obstacles mean that you cannot currently - without a great deal of trouble, move from the Gaza Strip to Egypt, nor along the Darien Gap between Panama and Colombia (even if the risk of being kidnapped in the latter region has lessened a little this decade, the heavily rainforested landscape means that no roads, including the great Pan-American Highway, currently cross the isthmus).

A tunnel between Tarifa and Tangier Med (or wherever. The precise route of the project has not been specified) would be the sort of reinvention of the wheel about which Jules Verne might have waxed lyrical 148 years ago (when Twenty Thousand Leagues... was published); a figment of fancy that would let you disappear under the skin of the planet in Andalusia and emerge a few minutes later on the continent of lions and Leptis Magna.

Of course, Libya's most celebrated Roman site drags the idea back to the hard truths of the early 21st century. Even if you took the train to Morocco from Spain, you would struggle to venture much further. Algeria, directly to the east, is a big question mark in terms of travel (the Foreign and Commonwealth Office currently warns of the high risk of terrorism), and Libya, beyond, is an obvious no-go zone - while progress south is limited by the undying sands of the Sahara Desert and the disputed status of Western Sahara (which Morocco claims, without international recognition).

Still, you could forge onwards into Morocco. To Tangier, to Rabat, to Marrakesh, to Fez. Or to Casablanca (which sits 240 miles south-west of Tangier Med), and wonder whether Bogart's famous line to Bergman - "Maybe not today, maybe not tomorrow, but soon, and for the rest of your life" - would have sounded quite as timeless if she had been attempting to board a delayed train towards Madrid rather than a small plane to Lisbon.