"Istanbul is a dynamically changing city, every year increasing in population," says Zeynep Buket, an engineer working with Turkey's transportation ministry. "We are in need of radical systems, and this radical system is a mass transit system."

The "radical system" city planners embarked on five years ago involved construction of a new subway tunnel beneath the Bosphorus Strait, the spectacular body of water that cuts this city in two. By the year 2025, engineers predict more than one million people a day will use the tunnel to travel between Istanbul's Asian and European shores.

"We will connect two continents, Asia and Europe," said Nusret Ilbay, one of the many engineers working on the $3 billion Marmaray Tunnel Project. He was standing on scaffolding, overlooking a gaping 30-meter deep hole that will one day be a subway station on the Asian side of the Bosphorus. A concrete wall is all that holds back a churning river of sea water.