The Bosphorus Strait, one of the most heavily traveled waterways in the world, cuts right through the Turkish city of Istanbul. Mammoth tankers on their way to the Mediterranean carry oil from the Caspian Sea region through the heart of the city, crossing paths with dozens of ferry boats. Concerned about accidents, explosions and terrorist sabotage, the Turks have imposed stringent safety rules, slowing the traffic. This has caused a long back up, and now some ships wait for weeks to make the journey through the Bosphorus. NPR's Ivan Watson reports from Istanbul.