The mine near the eastern town of Kamituga collapsed in torrential rains on Friday.
Local officials say most of the feared dead are young people and have called for two days of mourning.
Accidents are common in DR Congo’s mining industry, which has poor safety standards.
Video footage showed hundreds of people attempting to clear the rubble blocking the entrance to the mine, with some rescuers using shovels or their hands to move the debris.
The local governor of the province of South Kivu, Theo Kasi, said he was saddened by “the tragic death of 50 people, mostly young”.
However, the mayor of Kamituga, Alexandre Bundya, said: “We are not yet sure of the exact number” of people killed, AFP reports.
An eyewitness told AFP that heavy rains meant that “water was entering the three tunnels. When people were trying to get out, there was no way because the water was flowing hard ”.
Mr Kasi’s office added that it would “implement measures to prevent such incidents from happening again”.
DR Congo has rich reserves of minerals like cobalt, diamonds, copper and gold, but its people remain among the poorest in the world after years of conflict and mismanagement.