The answer has immense repercussions for the energy industry. If it is a lake, there are no obligations by countries that flank it to grant permits to foreign vessels or drilling companies. But if it is sea, there are international treaties obliging those countries to an array of permits.

The Caspian, one of the world's largest enclosed bodies of water, has become the center of a new power game involving the United States and Russia as well as its bordering countries, including Iran, over who should control the vast energy reserves under its depths.

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