Here we come to the most difficult and the most remarkable part of the whole project: the place where the continents are parted from one another by the sea, and a way must be found to stitch them together.
Fortunately, in the twentieth and twenty-first centuries mankind has gained a vast experience in the building of bridges and of tunnels beneath water.
Four ways of crossing water
The first way: the bridge. Suspension or cable-stayed bridges with long spans, resting upon deep piers that descend to the bed of the sea. It is excellent for shallower waters, but the deeper the water becomes, the more difficult and the more costly is the building of the piers.
The second way: the bored tunnel. Giant boring machines (TBMs) advance through the earth and rock beneath the sea, and as they advance they set prefabricated concrete linings in place. Its famous example is the Channel Tunnel, some 50 kilometres in length, of which close to 38 kilometres lie beneath the sea. This method is good for deep waters, but it is slow and expensive.
The third way: the immersed tube tunnel. The sections of the tunnel are made in advance, floated upon the water, and lowered into a trench prepared beforehand in the bed of the sea. For shallow to medium waters — from 5 to 40 metres, and in some designs as much as 100 metres — it is excellent; faster than the bored tunnel, and it can be made resistant to earthquake. Its only drawback is the need to dredge the seabed, which does environmental harm, though harm that can be controlled.
The fourth way: the floating tunnel. A tunnel which neither rests upon the seabed nor becomes a bridge, but remains suspended within the water itself, anchored by cables. It is the engineering answer to the most difficult conditions: very deep waters and geologically active regions. But I must be honest — this technology was proposed more than a century ago and to this day has nowhere been fully built; so it is a frontier solution of the future, not an option within reach.
In great projects these methods are often combined with one another. An example is the Hong Kong–Zhuhai–Macau bridge in China, which is a combination of bridge, artificial island and immersed tunnel. For the crossings of the global network too, the answer will probably be an intelligent combination of these four ways.
The four principal crossings
The Strait of Bab al-Mandab: in the shallower parts, an immersed tube tunnel beneath the bed is entirely practicable, and in some places a bridge could be built. But the deeper parts may require a bored tunnel.
The Strait of Hormuz: the same holds here — an immersed tube tunnel and, in places, a bridge in the shallower parts, and a bored tunnel where the water is deeper.
The Strait of Gibraltar: the most difficult and the most challenging, because its water is deep, its bed uneven, and it lies in a region prone to earthquake. The real studies made to this day — including those of 2025 — concentrate for the most part upon a twin bored tunnel. Here, perhaps, the floating tunnel may in the future become a serious option as well.
The Bering Strait: the water is relatively shallow, about 50 metres, and two small islands in the middle, like two stepping stones, make the work easier. But the chief challenge is the severe cold, the floating ice, and the great length of the crossing — about 88 kilometres. Here too, a combination of methods will probably be the best answer.
Besides these four crossings, the first artery, on its way to Australia, also passes over several other straits in the archipelago of South-East Asia; their exact location and the manner of crossing them will be determined and designed by engineers after study.
Existing tunnels: use, not duplication
What is to be done with existing structures? First it must be seen whether they are compatible with the new standards and can be brought up to date. If they can, we shall make use of them. And if they cannot bear the new standard, a new structure will be built beside them and the old structures will remain for their original purpose. In this way, no capital is wasted and nothing is needlessly destroyed.
The decision lies with the specialists and with the people
What I have set out in this chapter is a picture of the engineering possibilities of today, not a final verdict. Which method for which crossing? This only specialist engineers, after exact studies, are able to answer. And above the technical decision stands the human decision: all these plans must be put to the vote in the general assemblies of the Consortium.
But this is what gives ground for hope: the man of today holds in his hand the means of crossing these waters. What is lacking is not technology, but will and cooperation.