Turkey has opened the world's first underwater rail link between two continents, realising a project dreamt up more than a century ago.
The engineering feat spans eight miles to link Europe with Asia some 60m below the Bosphorus Strait. Called the Marmaray, it will carry subway commuters in Europe's biggest city and eventually serve high-speed and freight trains.
"Today we are realizing the dreams of 150 years ago, uniting the two continents and the people of these two continents," Erdogan said at the opening, which coincides with the 90th anniversary of the founding of the modern Turkish Republic.
The 5.5bn lira £1.7bn) tunnel is one of Erdogan's "mega projects," an unprecedented building spree designed to change the face of Turkey, which also includes a 50-km canal to rival the Suez that would render half of Istanbul an island, an airport that will be the world's busiest and a giant mosque atop an Istanbul hill.
Atomic power stations are on the drawing table. A third bridge over the Bosphorus, whose construction has already felled 1 million trees, is under way.
The plans have fired up Erdogan's opponents who dub them "pharaonic projects," symptom of an increasingly authoritarian style of government, and warn of environmental catastrophes in one of world's most earthquake-prone nations.
Erdogan argues his policies meet the needs of a rapidly expanding and increasingly affluent population.
"Roads are civilisation," he said last week. "Our values recognise no obstacle for roads. If a mosque is where a road will go, we will tear down that mosque and build it elsewhere."
Erdogan has called the Marmaray the project of the century and says it fulfils an age-old "dream of our ancestors".
Plans for a rail tunnel below the Bosphorus date to at least 1891, when Ottoman Sultan Abdulhamid, a patron of public works whom Erdogan frequently evokes, had French engineers draft a submerged tunnel on columns that was never built.
Today, the Marmaray is an immersed tube set in the seabed built by Japan's Taisei Corp with Turkish partners Nurol and Gama. The bulk of financing came from the Japan Bank for International Cooperation.
"Japan and Turkey are the two wings of Asia. Let us dream together of a high-speed train departing from Tokyo, passing through Istanbul and arriving in London," said Japanese Prime Minister Shinzo Abe, who attended the opening.
The Union of Chambers of Turkish Engineers and Architects warned the Marmaray set on a silty seabed 20km from the active North Anatolian Fault is at risk in case of a large earthquake, which geologists predict may strike within a generation.
But Yildirim described the Marmaray as the "safest structure in Istanbul," its free-floating structure designed to withstand an earthquake with a magnitude of 9. Interlocking floodgates would seal off each section.
Some 17,000 people were killed in 1999 when a 7.6 magnitude quake struck the western city of Izmit.
The Marmaray will reduce car traffic by 20 per cent in Istanbul, among the world's most congested, when it eventually carries 1.5 million people a day.
Murat Guvenc, director of the Urban Studies Research Centre at Istanbul Sehir University, said the tunnel will essentially shrink Istanbul, a sprawling metropolis of 15 million people.
"The historical peninsula has remained intact for 25 centuries, like the eye of the storm, because of the natural barriers of the Golden Horn and Bosphorus waterways," Guvenc said. "The Marmaray removes those boundaries."
Construction of the tunnel on the European side yielded a Byzantine port with more than 13 shipwrecks and thousands of other relics that date back as far as 8,500 years.
The finds nearly doubled the project's duration and prompted UNESCO, the United Nations' cultural arm, to voice concern about threats to the peninsula, a World Heritage site.
The government will open an "archaeological park" at the Yenikapi subway station to showcase relics. Station walls are decorated in a Hellenic theme with amphoras and galleons.
"Had it been up to the archaeologists, this project would have never finished," Yildirim said.