Lake Baikal is the deepest and oldest lake in the world, it contains more freshwater than any other body of water, and it's ridiculously clear. The water is so pure that this area of Russia is the only one where the guidebooks don't call the tap water dodgy.
Most people get here by going to Irkustk, down the river. It's a nice city with many cool old wooden buildings, parks, and churches. The easiest way to the lake is to find a minibus (a large van) going to Listvyanka, and when it fills up, it leaves for the hour drive. Most of the drive gives you a view of a dense mix of birch and conifer trees. Listvyanka is a much smaller town centered around tourism on the edge of the lake. The coolest thing about the lake is that is has the only freshwater seals in the world - and they are very cute! We didn't see any in the wild, but we saw a show (a la Sea World) where two seals do a whole bunch of tricks like play soccer, play instruments and paint.