Lake Baikal is the central feature of a massive rift zone formed between the Siberian craton and several microplates. Although controversial, a leading theory for rift onset was the oblique collision of India with Asia, which rotated China counterclockwise and began to open the basin. The northern end of Baikal is more mountainous than the south, and peaks in Zabaikalsky top out at more than 2,500 meters. Jutting out into the lake is the Holy Nose Peninsula, which rises abruptly 1,422 meters above the water. Best estimates for the age of the lake are 25 to 27 million years old, making Baikal by far the planet´s oldest lake.
The rift continues to open and to generate large earthquakes. A magnitude-7.5 quake on January 12, 1862, around the mouth of the lake´s biggest tributary, the Selenga River, dropped 200 square kilometers of the coastline by as much as two meters and made a new bay, now one of the lake´s best birding spots.
Baikal bottoms out at 1,637 meters below the lake´s surface, with an average depth of about 750 meters. Below the lake bottom is an additional 7.5 kilometers of sediment in the deepest parts of the basin. Because of the great depth and great length, Baikal contains more water than all of the Great Lakes combined, roughly one-fifth of Earth´s freshwater.
The lake´s depth and age have made it an evolutionary hot spot. More than 2,500 species, around 2,000 of which are endemic, dwell in its waters. Species include a 40-centimeter long flatworm, 255 species of amphipods and the only freshwater seal, the nerpa. Enigmatic in origin, nerpas may have arrived at Baikal from the north during the Plio-Pleistocene. One theory holds that the seals ascended either the Angara/Jenisei or Lena rivers to the lake, but evidence has not been found for their exact route.
Returning to Ulan Ude, you can catch a train (assuming you can navigate the astoundingly unhelpful information people at the station) that will take you around the south end of Baikal to the fine little village of Sludyanka. Slud is the Russian word for mica. You can find immense mica specimens, as well as dozens of other beautiful minerals, at the local mineralogical museum, a 20-minute walk from the train station. The museum gift store, funky at its best, sells many unusual specimens, including charoite, a lavender-colored silicate mineral found in only one location in Siberia. You can also buy aquamarine apatite crystals ... though the owner of your hostel may laugh at you (as mine did) because you paid $12 for a two-inch-long crystal and he uses a bigger one to prop open a door.