A giant new canal for the world’s biggest ships is the most ambitious engineering plan yet proposed by Turkey’s President Erdogan, whose massive infrastructure projects have already changed the face of his country. The proposed waterway would slice through Istanbul, creating in effect a second Bosphorus, the busy shipping lane that is now the only outlet from the Black Sea. The president himself has called the project 'crazy'. But he says it would 'save the future of Istanbul', easing traffic in the Bosphorus and reducing the risk of a terrible accident there. The plan has met a storm of opposition. Istanbul’s mayor says it would “murder” the historic city. Critics claim the canal would be an environmental disaster, cost billions of dollars that Turkey can’t afford – and provoke severe tensions with Russia, which is determined to preserve existing rules on traffic into and out of the Black Sea. Tim Whewell reports from a divided city. He sails down the Bosphorus with a pilot who knows all its twists and turns, joins a marine biologist diving for coral in the Sea of Marmara, meets a woman whose Ottoman-era mansion was wrecked by a ship, has tea with a former admiral who was arrested over his opposition to the project, and visits the tranquil forests around the city, now threatened by development. Will the canal still go ahead? Who would lose – and who would benefit?