Immersed tube
An immersed tube (or immersed tunnel) is a kind of undersea tunnel composed of segments, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road and rail crossings of rivers, estuaries and sea channels/harbours. Immersed tubes are often used in conjunction with other forms of tunnel at their end, such as a cut and cover or bored tunnel, which is usually necessary to continue the tunnel from near the water's edge to the entrance (portal) at the land surface. Construction. The tunnel is  made up of separate elements, each prefabricated in a manageable length, then having the ends sealed with bulkheads so they can be floated. At the same time, the corresponding parts of the path of the tunnel are prepared, with a trench on the bottom of the channel being dredged and graded to fine tolerances to support the elements. The next stage is to place the elements into place, each towed to the final location, in most cases requiring some assistance to remain buoyant. Once in position, additional weight is used to sink the element into the final location, this being  a critical stage to ensure each piece is aligned correctly. After being put into place, the joint between the new element and the tunnel is emptied of water then made water tight, this process continuing sequentially along the tunnel. The trench is then backfilled and any necessary protection, such as rock armour, added over the top. The ground beside each end tunnel element will often be reinforced, to permit a tunnel boring machine to drill the final links to the portals on land. After these stages the tunnel is complete, and the internal fitout can be carried out. The segments  of the tube may be constructed in one of two methods. In the United States, the preferred method has been to construct steel or cast iron tubes which are then lined with concrete. This allows use of conventional shipbuilding techniques, with the segments being launched after assembly in dry docks. In Europe, reinforced concrete box tube construction has been the standard; the sections are cast in a basin which is then flooded to allow their removal. Advantages and disadvantages. The main advantage of an immersed tube is that they can be considerably more cost effective than alternative options – i.e., a  bored tunnel beneath the water being crossed (if indeed this is possible at all due to other factors such as the geology and seismic activity) or a bridge. Other advantages relative to these alternatives include: Disadvantages include: Tubes can be round, oval and rectangular. Larger strait crossings have selected wider rectangular shapes as more cost effective for wider tunnels. Examples. The first tunnel constructed with this method was the Shirley Gut Siphon, a six-foot sewer main laid in Boston, Massachusetts in 1893. The first example built to carry traffic was the Michigan Central Railway Tunnel constructed in 1910 under  the Detroit River, and the first to carry road traffic is the Posey Tube, linking the cities of Alameda and Oakland, California in 1928. The oldest immersed tube in Europe is the Maastunnel in Rotterdam, which opened in 1942. The Marmaray Tunnel, connecting the European and Asian sides of Istanbul, Turkey, is the world's deepest immersed tunnel at below sea level; it is the first rail link crossing the straits. Construction began in 2004 and revenue service began in 2013. The tunnel is long overall, of which were constructed using the immersed tube technique. Currently the longest immersed tube  tunnel is the tunnel portion of the Hong Kong–Zhuhai–Macau Bridge, completed in 2018. The HZMB tunnel is set at a depth of below sea level. Its length will be surpassed by with the completion of the Shenzhen–Zhongshan Bridge in 2024. The SZB project includes a immersed tube which also will be the world's widest immersed tube, carrying eight traffic lanes. Prior to the completion of the Marmaray and HZMB tunnels, the Transbay Tube in San Francisco Bay, completed in 1969, was the world's deepest and longest immersed tube, at below water level and long. The  length of both the HZMB and SZB will be surpassed by the Fehmarn Belt Fixed Link connecting Denmark and Germany when it is completed, at an as-designed long. Construction started on 1 January 2021.