The daunting 40km crossing has been the subject of many site investigations and surveys by Moroccan state-owned company SNED and its Spanish counterpart SECEG since it was first mooted in the late 1970s. SNED engineer geologist for site data Ali Bahmad says: 'We have already carried out 40 investigation campaigns including bathometry, side scan sonar, geophysics, seismic and dredging, short borehole coring and deep boreholes.' As a result of their findings the likely route between Punta Paloma, 40km west of Gibraltar on the Spanish side and at Punta Malabata, near Tangier, has had to be revised more than once.
'When we started with seismic surveys and dredging we thought the whole route would be flysch, similar to the rock on both sides of the Strait. Flysch is generally an alternation of hard rock layers such as limestone or sandstone and clay that means it's impermeable, ' he says.
Had this been the case the tunnel could have been built with an overburden of about 100m.
With a tunnel angle of 2.4%, the slope would have been similar to the Channel Tunnel and the Seikan Tunnel in Japan.
But two pockets of breccia within the flysch in a 5km section of the Straits have lead to the depth of the tunnel being revised more than once in an attempt to get underneath it. The fear is that the breccia could contain pockets of sand that could provide a porous link to the sea.
The extent of the problem was difficult to determine by seismic surveys, which refl ected back from the hard crust of Quaternary limestone on the breccia.
But Bahmad says: 'When we started dredging and found more recent deposits instead of flysch we decided on a short boring programme with the British Geological Survey. It involved a submerged rock drill tele-operated from a dynamic positioning vessel. The result was we found these Quaternary deposits were more than 5m thick.' In 1997 a deep drilling programme showed the two deposits were more than 100m deep. So in 1998 to 1999 a second deep drilling campaign put boreholes down to test a new lower alignment, this time to 200m.
But rather than passing through into competent strata, the drillers discovered the breccia was still present beyond 200m depth. The tunnel route had to be altered again, to run another 100m deeper into the seabed.
At the time of GE's visit, contractor Seacore was drilling beyond 300m, which it claims is deeper than anybody has probed before in the Gibraltar Straits, in another attempt to locate the end of the breccia. But at 325m, the bottom had still not been reached.
However, for the tunnel to go any deeper would mean its length being increased to flatten out the slope angle for the rail tracks.
So the planners are looking at a shallower route that would have to go through the breccia after all.
Now investigations are focusing on pinpointing any sand deposits.
Various costs for the project have been quoted, from £2bn to £6.8bn, and work could begin in 2008 if the project gets the go-ahead.