During ice-diving, in a frozen lake under a often several centimeter-thick ice cover. A device is normally used. Ice diving under ice is considered an extreme sport.
Very dangerous is that the dive can not simply be interrupted by surfacing on the surface and on the other hand that you can cool quickly.
Lake Baikal is a lake in Siberia, Russia. At 1642 meters, it is the deepest and with more than 25 million years the oldest fresh water lake on earth.