Istanbul was once Constantinople, co capital of the Roman Empire and then sole capital of the Byzantine, a true nexus between Europe, Asia and Africa. As the centre of a massive trading network, and a port, Constantinople had many ships visit... and many ships sink for one reason or another.
The city is now Istanbul, and it's used to finding the remnants of history any time someone digs. Nevertheless, when tunnels were to be dug under the Bosporus Strait archaeologists had to be called in, and explorations have uncovered a mass of shipwrecks. NPR.org cites a figure of thirty six from Ufuk Kocabas, saying "This is the world's biggest shipwreck collection ever found." The ships and their contents are now being preserved and studied in a project which will take years.