The idea of building the crossing using a floating tunnel or a seabed tunnel was thoroughly evaluated.
Solutions involving underwater tunnels (floating) and seabed tunnels (beneath the seafloor) were thoroughly investigated when initially considering what type of stable crossing to build. Both scenarios were later discarded for the following reasons.

Seabed tunnel

• vulnerability to seismic activity resulting from direct proximity to several active faults in the stretch of sea concerned, particularly regarding deformation or potential rupture of the structure caused by ground movements of even only modest entity, thereby failing to ensure that the tunnel can remain watertight;
• considerable depth of the tunnels (up to 250 meters below sea level), requiring the construction of extremely long access ramps; given the restrictions imposed by the related gradients – essential above all for rail runability -, it would be necessary to build approximately 35 km road tunnel and 50 km of railway tunnel to cross a stretch of sea little more than 3 km wide;
• longer travel times;
• difficult access for rescue or evacuation operations in case of accidents or natural disasters;
• exposure to sabotage.

Floating tunnel

• major impact on the marine environment: the placement of floating tubes at shallow depths (40-50 meters) and their anchorage to the seabed at depths exceeding 200 meters would cause substantial disturbance to flora and fauna (difficult to mitigate), as well as to sea currents;
• uncertainties about structural safety and ability to meet performance requirements;
• lack of previous projects in areas with high seismic activity and significant sea currents, leading to uncertainty regarding construction technologies, high risks during construction, substantial costs, and difficulties in predicting outcomes;
• maintenance uncertainties;
• impact hazards related to sinking ships;
• exposure to sabotage.