IN January of this year, an interesting phenomenon was observed on Lake Bohinj in the Julian Alps. This lake, in the extreme north-west of Yugoslavia, is a typical alpine lake, its basin having been hollowed out by a diluvial glacier. Its altitude is 523 m., it is about 4 km. long and 1 km. wide. In winter the lake is thickly frozen over and last winter was no exception. Acute tangential tensions always set up in the ice crust and find adjustment in various cracks which extend over the entire width of the lake in straight or broken lines, as the case may be. As a rule, the cracking of the ice is accompanied by a powerful detonation which can be heard distinctly for a distance of several kilometres.