Cleaning up the Bosphorus
6 March 2012 Last updated at 13:55 GMT
Istanbul is one of the world's oldest cities, and with 15m inhabitants one of the most crowded.
That has put immense environmental pressure on the Bosphorus, the narrow strait that divides the city and connects the Black Sea to the Mediterranean.
For centuries the Bosphorus has been Istanbul's rubbish dump - now some of its citizens are making an effort to clean it up.
The BBC's correspondent Jonathan Head reports.