(PT, give us some sense of scale.) It's the largest body of fresh water on the planet, with about a fifth of the world's fresh water, which is more than all the Great Lakes combined. (I've also been reading that Baikal is home to a number of unique species. Are they under threat of any kind?) They are from different angles. One is just pollution, another is climate change. Siberia is a cold place but the temperature is rising and it's accelerating. (Now we'll talk to James Rodgers, traveling with the expedition on Baikal). To give you an impression, it's difficult to believe you're standing on a lake, it's more like you're at sea. (Why are people just determining how deep the lake is now?) I think because it's such a vast and inaccessible body of water. This is the same team that planted the Russian flag at the bottom of the North Pole last summer. They're trying to establish the depth of the lake and they're not satisfied with their results today. (Are there political considerations to this expedition?) I think there may be some political consideration here. it's quite clear there was in last year's expeditions. This is a new Russia which can mount difficult scientific expeditions. (PT, presumably it gets dark at the bottom of Baikal.) Yes this is more than a mile down and you can't see anything below 500 meters.