At least six people have died after a cassiterite mine collapsed in Burundi, 120km (75 miles) north of the capital.
Three other people injured in the accident have been taken to hospital, and another four are missing.
The authorities in Kayanza province, where the accident happened, told the BBC the group were digging illegally.
A BBC reporter says despite Burundi's mineral wealth there are few official mines and even they use crude methods of extraction and safety is poor.
Most of the small mines found in the west and north of Burundi are exploited by villagers using hoes and ploughs.
Burundi is one of the world's poorest nations and is emerging from a brutal 12-year civil war which shattered its economy.
Mobile phone mineral
The BBC's Prime Ndikumagenge in the capital, Bujumbura, says mining accidents are common at this time of year, when the first rains arrive.
Cassiterite is the main ingredient for tin.
The mine fields in the northern province of Kayanza are also rich in colombo-tantalite ore, or coltan, used to make mobile phones.
Victor Ntakirutimana, the administrator of Kabarore district in Kayanza where Wednesday's collapse occurred, told the BBC many villagers do not mind risking their lives to get even a small amount of the mineral as it is so valuable.
Residents also say it is the miners' belief that when a big number of people perish inside a mine it is a sign that the mine is rich - so survivors continue to dig it even more eagerly, our correspondent reports.