The workers have been trapped since early Tuesday morning after a mudslide flooded the shaft where they were working, an alliance of South African miners' unions said. They are believed to be about 800 meters underground.
Rescue workers pumped water out of the shaft on Wednesday, while others tried to drill a hole to the area where the miners are feared to be trapped, in order to make contact with them or get signs of life.
The diamond mine is in the city of Kimberley in central South Africa.
Last year, 41 miners died in work-related accidents in South Africa. This is a record low and a decrease from hundreds per year in the 1990s and early 2000s.
