Around noon, the storage basin for flotation residue at a lead, zinc, copper and silver mine ripped open, releasing 450,000 m³ of sludge that killed 488 as it swept through the village of Sgorigrad below and continued for another 7 km until reaching Vratza, where the central square was covered by 20 cm of sludge. A new basin was built on the opposite valley slope, and the mine closed operations in 1996.

The basin had been built in 1960 at the same time as the flotation plant by damming a valley using mining tailings. The streams flowing at the bottom of the valley had to be diverted and channelled. Heavy rains combined with a break in the stream bypass canal (though no direct link of cause and effect could be established) caused the basin to overflow, quickly leading to erosion failure.

In the days following the accident, Soviet authorities announced a death toll of 107 and sought to prevent information circulation by ordering the police to confiscate all photographs of the disaster. 40 years later, the Sgorigrad town hall raised the victim count to 488.