Infographic: Dan Marsiglio
Under the city From both sides of the strait, 26-foot-wide tunnel-boring machines chew through the bedrock that underpins Istanbul. Then workers reinforce the tunnel — installing 13,000 concrete rings, one at a time.
Under the Bosporus Builders inject grout into the seabed to stabilize it, and dredgers dig out a trench 40 feet deep. Barges bring tunnel segments — up to 440 feet long and weighing 18,000 tons — and workers guide them down on cables using GPS and ultrasound. Water pressure seals the segments, and the dredgers backfill the seabed.
