Running under the Bosphorous Strait, the $1.1 billion, 8.5-mile tunnel connects the European and Asian sides of Istanbul.

It's the world's deepest submerged tunnel, and the first to connect two continents.

"It's been a dream to connect the two sides by tunnel since as long as the Ottomans have controlled the city," said KPMG's Kjorstad.

It opened in October 2013 and has a capacity to carry 1.5 million people a day -- making the intercontinental crossing in about four minutes.