When digging into any extraordinary construction project, a few questions immediately poke their heads out of the sand: Which proposal ranks as the biggest, the deepest or the most hazardous to build? With underwater tunnels, these questions defy simple answers. Cities and countries constantly commission new projects. As for vital stats, the devil is between the details and the deep, blue sea.
For example, the Seikan Tunnel connecting the Japanese islands of Honshu and Hokkaido currently holds the record for the longest and the deepest underwater rail tunnel. Japan began planning it after a 1954 typhoon sank five ferry boats in the dangerous Tsugaru Strait, killing 1,430 people [sources: WGBH].
Completed in 1988, the Seikan Tunnel stretches 33.5 miles (54 kilometers) and reaches a depth of 787 feet (240 meters), but its 14.5-mile (23.3-kilometer) undersea portion is dwarfed by that of the Channel Tunnel, or Chunnel, between the United Kingdom and France. Finished in 1994, the Chunnel's underwater portion accounts for 24 of its 31 total miles (38.6 of 50 kilometers) but plunges only 246 feet (75 meters) deep [sources: ASCE; Chan; Wise].
As far as the Turks are concerned, both tunnels are all wet compared to their $3.3-billion Marmaray Tunnel finally open to the public in 2013. Its 8.25 miles (13.2 kilometers) of railroad passage -- including a 4,600-foot (1,400-meter) stretch across the Bosporus seafloor -- connect Istanbul's Asian and European halves, making it the first rail tunnel to connect two continents [sources: Sweeney; Wise].
What's so great about a sub-mile undersea tunnel compared to the multi-mile Seikan and Channel tunnels? It's a difference of approach: Whereas its predecessors respectively blasted and bored passages through solid rock, the Marmaray Tunnel was assembled, piece by piece, in a trench along the Bosporus' bottom, which makes it the longest and deepest immersion tunnel ever built. Engineers chose this solution, which employs preassembled sections connected by thick, flexible, rubber-reinforced steel plates, to better contend with regional seismic activity [sources: JR-Hokkaido; Sweeney; Wise].
For a time, cultural and historical artifacts found throughout Istanbul's old city slowed progress on the Marmaray Tunnel excavation, so the 2.2-mile (3.6-kilometer) Øresund Tunnel connecting Sweden and Denmark remained the largest immersed-tube tunnel ever built. Contractors constructed it from 20 elements measuring 577 feet (176 meters) apiece, each assembled from eight smaller, 72-foot (22-meter) sections [sources: Landler; Marmaray Project; PERI GmbH; Sweeney].
Between immersed tunnels like Marmaray and Øresund, and bored tunnels like the Chunnel, we've just about covered the waterfront. But let's delve a little deeper into each and examine another tunneling method used since the early 19th century.