The knocking sound came at around 2 a.m. Monday in response to rescuers' knocking at a 277.9-meter deep hole that had been drilled on Sunday night, rescuers said, noting this signals sign of life underground.

The hole can supply ventilation to the area where the miners might be trapped, and it can also be used to send down water and food to the trapped miners.

The flooding occurred on Tuesday, when 45 miners were working in a pit of the Hengtai Coal Mining Co. in Boli County of Qitaihe City, Heilongjiang Province. Nineteen miners managed to escape.

On Saturday, four of the 26 miners who were trapped underground were confirmed alive. But only three were brought to the ground that day. The last one of the four was confirmed dead early Sunday and his body was retrieved around 3:30 a.m. Sunday.

The provincial government ordered the mine to stop production in 2007. But the owner restarted production without permission on Aug. 16, according to the local work safety bureau.

The accident happened after workers mistakenly drilled into a neighboring mine.