Baikal's age is 25–30 million years: it is one of the oldest lakes. It is unique among large, high-latitude lakes, because its sediments have not been scoured by overriding continental ice sheets. U.S. and Russian studies of core sediment in the 1990s gave a detailed record of climatic variation over the past 250,000 years. Longer and deeper sediment cores are expected soon. Lake Baikal is the only confined freshwater lake in which evidence of gas hydrates exists.