Seen as one of the most ambitious undersea projects in the world, the tunnel plan featured a double-rail track and additional service line stretching 38.5 kilometers, of which 28 kilometers would run under the Mediterranean at a maximum depth of 475 meters.
Working through a joint committee, two state-run companies, Spain's SECEGSA and Sned of Morocco began feasibility studies which would, over the next four decades, involve multiple tests and preliminary drilling work.
After considering various scenarios, they agreed on a model inspired by the Channel Tunnel between Britain and France, that would run from Punta Paloma near Tarifa in southern Spain to Malabata near Tangiers in Morocco.
In linking the two countries' rail networks, the tunnel would "boost the European and African economies", said Claudio Olalla, an engineer and professor emeritus at Madrid's Polytechnic University who has been working on the project for some time.
According to SECEGSA, the tunnel would, in the medium term, enable the annual passage of more than 13 million tons of goods and 12.8 million passengers, which could "significantly contribute" to the economic development of the western Mediterranean.
Spain is already the main trading partner of Morocco, which exports a large part of its own production - predominantly agricultural - to the European Union.
But the Gibraltar Strait, through which 100,000 ships transit every year, is already heavily congested slowing the transit of goods between the two countries.
-Why revive plans now?-
The project stalled in recent years due to budget cuts in Spain following the 2008 financial crisis and a succession of diplomatic spats with Morocco.
But relations have improved since March 2022 when Madrid ended a nearly year-long diplomatic crisis by reversing decades of neutrality on the Western Sahara conflict to back Morocco's position.
That has seen the two nations reset their strategic partnership, with Madrid unblocking part of its 2023 budget to fund a new study that will constitute "the definitive step needed to start construction".
The two sides discussed the vast project during a summit in Rabat on February 2.
"We are going to give a new impetus to studies into the Gibraltar Strait link project" said Transport Minister Raquel Sanchez in a statement announcing that the joint committee grouping SECEGSA and Sned would resume talks.
-What obstacles stand in the way?-
The main problem is technical: the Strait of Gibraltar is located in a fault zone between the Eurasian and African tectonic plates with a complex geological makeup that includes an unstable clay section that is constantly pounded by intense underwater currents.
"The soil quality is pretty poor. Nothing like the limestone they have underneath the Channel," Olalla told AFP, referring to the body of water separating Britain from France.
"The technical conditions are very challenging, far beyond those of any other tunnel in the world," he said.
And that is likely to weigh heavily on the cost, which has never been clearly laid out.
"Technically, these obstacles are not insurmountable but their economic viability is a matter of concern," he said.
There are also political issues given the frequent instability of ties between Madrid and Rabat as well as the potential reluctance of other European countries that fear a surge in migration -- which the tunnel's promoters have insisted won't happen.
All of which makes the project unlikely in the short-to-medium term, said Olalla.
"I think the project will go ahead one day, but it won't happen any time soon."