The scope and complexity of the project is being compared to the Panama Canal and the Chunnel. The Swiss firm Lombardi Engineering won the contract in 2006 for the design of the tunnel which is currently not scheduled to be completed until 2025. As far as the geologic challenges go:
“It is muddy and unstable right at the seabed, unlike the harder surface at the bottom of the English Channel,†AP says. “Farther down are huge pockets of debris from tectonic slides – a cocktail of sand, stones and mud that make for a digger’s nightmare.â€
Add to that the fact that the tunnel will be crossing the African and European tectonic plates and the tremendous currents in the straits not to mention water pressures of up to about 187-ksf. So you might expect a large price tag, and you would be right.
The overall cost of the project was first estimated between US$4.43 billion and US$14.77 billion (3-10 billion euros). As of last year, the cost may reach as high as $15.6 billion. “The Moroccan and Spanish governments still have not decided whether the large infrastructure project will be operated by private capital or by the two states…
Source: ThomasNet by way of Geology.com