A multi-billion-dollar pipeline carrying Russian gas under the Black Sea to Turkey has been officially opened in the Turkish port of Samsun.
Turkey is keen to develop the pipeline project further
It is the world's deepest undersea pipeline - in places, "Blue Stream" reaches a depth of more than two kilometres (1.5 miles).
The Turkish prime minister hosted the Russian and Italian leaders in Samsun.
The project has been plagued by disputes and the first gas flowed through the pipeline two years ago.
So this project may well set another record - as one of the most delayed opening ceremonies ever.
Russia's President Vladimir Putin said there was now an opportunity to build another section of the pipeline, to pump gas across Turkey and export it to southern Italy, the south of Europe and Israel.
But the project has suffered from price disputes, technical problems and allegations of corruption.
BLUE STREAM PIPELINE
Pumps: 3.2 billion cubic metres of gas a year
Capacity: 16 billion cubic metres of gas a year
Deepest pipe: 2.15km
Total cost: $3.2bn (£1.8bn)
Firms: Gazprom (Russia); ENI (Italy); Botas (Turkey)
Blue Stream was originally meant to supply Turkey's domestic markets, providing more than 60% of the country's natural gas needs.
But Ankara now sees the pipeline as a step towards becoming a key player in world energy markets, the BBC's Sarah Rainsford reports from Istanbul.
Together with Italy, Turkey is also lobbying for an oil pipeline to run alongside it.
Turkish Prime Minister Recep Tayyip Erdogan said described Blue Stream as the first link in the energy chain across Turkey.
Italian Prime Minister Silvio Berlusconi joined him and Mr Putin at the inauguration ceremony.
Turkey no longer needs all the gas it signed up for. It cannot refuse it, so it is pushing to extend the pipeline south to the Mediterranean, taking extra gas on to Israel.
An oil pipeline running parallel to Blue Stream would offer a major new oil route from Central Asia, bypassing the Bosphorus Straits, now so dangerously overcrowded.
There was concern when Turkey first signed up to the project that the country would become dangerously dependent on Russia for gas.
By developing these plans, Ankara hopes Moscow will become just as dependent on Turkey as a transit route.