By Danny Wood
BBC News, Madrid
Until now, crossings between Spain and Africa have always been by sea
Spain says a feasibility study for an undersea tunnel to connect Spain and Morocco is in the final stages.
If the project goes ahead and construction begins, trains carrying both passengers and goods are expected to start using the tunnel in 2025.
The tunnel would be 40km long and pass 300m under the Mediterranean Sea.
The undersea link would unite North Africa and Europe for the first time since the continents separated more than 200 million years ago.
Swiss engineers are finalising a feasibility study that will determine whether this underwater connection is technically possible.
However, Angel Aparicio, president of the Spanish government agency co-ordinating the project says building the tunnel presents difficulties that may not be possible to overcome.
"The material here is not compact enough to allow an initial excavation.
"It is clay with rock and so it is not as compact as it is in the rest. As we have a lot of water we have a very high pressure and we are not sure whether we could go through with the tunnelling," he said.
"Those are the difficult questions."
Years of talk
If construction goes ahead the tunnel will take 15 years to build and cost at least $8bn (£4bn).
The Spanish and Moroccan governments see the tunnel as part of a new Mediterranean transport hub for passengers and goods.
Others are not so sure. The prospect of a physical connection between their country and the poorest continent in the world is alarming to some Spaniards.
Others are sceptical about this ambitious scheme ever being completed.
Spain and Morocco have discussed bridge and tunnel plans for more than 20 years.
However, this time the project has support from the European Union and the possibility of funding from the World Bank.
If the feasibility study is positive, work on the tunnel could start in 2009.