South African authorities have detected “movement” underground at a shuttered gold mine where they believe at least 31 illegal miners died in a suspected gas explosion last month, raising the very slim possibility that there may be survivors, officials said.

Officials also said it’s likely that there were more illegal miners underground than initially thought and the death count will be higher than 31. But a search operation at the disused Virginia gold mine in the central city of Welkom has not yet been launched because of the dangerously high levels of methane gas still present in the mine, which means there could be more explosions.