http://www.turkeytravelplanner.com/go/Istanbul/Sights/Bosphorus/index.html: The site offers information: The Bosphorus is the 32 km (20-mi)-long strait which joins the Sea of Marmara with the Black Sea in Istanbul, and separates the continents of Europe and Asia. It's great for a half-day cruise north toward the Black Sea. You can return to Istanbul by land along the European shore and see all the sights.