Russia's Lake Baikal, the world's deepest freshwater lake, has extremely high pollution levels, President Vladimir Putin warned Friday while visiting the Siberian lake. Lake Baikal is a UNESCO World Heritage site, the world's deepest lake at 1,700 metres (5,580 feet) and also the oldest at 25 million years. In 2009 he went down to the bottom of Lake Baikal in a mini-submarine.