Last week the FASTER cable, a new transpacific network link with a theoretical peak capacity of 60 Tbps (that's 60 million megabits), landed in Japan. FASTER is a joint project between Google and several ISPs to provide, well, faster Internet speeds between the two regions. The album (with pictures courtesy of http://gigazine.net/news/20150615-kddi-faster/
) give you a quick overview of how such a cable is installed.
PS: see how this was done in 1956 for TAT-1, the first modern transatlantic telephone cable (with capacity for 36 phone calls, or about 0.2 megabits): http://atlantic-cable.com/Cables/1956TAT-1/index.htm