It is currently making its first pass down with a 12.5-inch diameter hole to the roof of the 50-square-meter (500-square-foot) refuge at Level 100 (100 meters above sea level).
Mr Jessop said the vault was found after archaeologists dug an exploratory hole in the internal floor of the mausoleum and discovered a brick-built roof.
Earlier Wednesday, a fifth hole drilled down into the mine found just six inches of open space left between the roof and rubble in an 8-foot-high tunnel, said Jack Kuzar, a district manager with the federal Mine Safety and Health Administration.