Eight miners died after being buried under mounds of earth that collapsed on them in an open-pit copper mine in Zambia on Wednesday, police said.

One miner was missing and another two survived, provincial police commissioner Peacewell Mweemba said. State media reported that six of the miners who died at the mine in Chingola, a city in Copperbelt Province, were from the same family.

The victims were not employees of the mining company but part of a group who had been searching for copper at the mine without permission, a common phenomenon in Zambia.

It’s the latest in a series of tragedies involving informal miners in copper-rich Zambia. This month, 10 informal miners died in a collapse in Mumbwa in central Zambia. Nine men died at a quarry near the capital, Lusaka, in August when a huge pile of earth collapsed on them.

Last December, more than 30 informal miners died at another open-pit mine in Chingola when heavy rain caused landslides that buried them in tunnels they were working in.

Zambian President Hakainde Hichilema recently said the number of miners who had died in accidents was “unacceptable.”