If there is the most epic metro line you should use at least once in your life, this is definitely Marmaray in Istanbul. Of course, it is not the only one underground line that runs beneath the strait. However, Marmaray is the only one connecting two continents! Concerning everything else, this is just a usual metro line – apart from the fact that 1,4 km of the tunnel is situated under the sea bed.
In fact, the construction of this tunnel began back in 2004. However, it was opened just in October 2013. The main reason for such a long time is not procrastination: while making stations on the European side, workers excavated multiple historical findings, some of them being up to 8,000 years old. Every time something was found, archaeologists stepped in, and the construction had to be paused.
The underwater part of the tunnel was built on land, piece by piece, and then just drown into the right place. Overall, it consists of 11 sections, firmly attached to each other in order to avoid leakages. The depth of the tunnel is almost 60 meters below sea level: 55 meters of water and 4,6 meters of earth. That is why it bears the name of the deepest undersea immersed tube channel in the world.
Apart from making the tube fully water-isolated, constructors took into account the location of the tunnel. Being close to the seismic zone (all borders between continents are seismic), Marmaray can easily stand earthquakes with a magnitude up to 9 degrees.
By now, there are just 5 stations in this line (Kazlicesme, Yenikapi, Sirkeci, Uskudar and Ayrilik Cesmesi), but the plan is to add 36 more. The most “interesting” part of the line is between Sirkeci and Uskudar, where the actual underwater tunnel is. Traveling between these two stations takes just 4 minutes – much less then traveling by ferry or using the bridge.