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Crews and experts are still looking into the potential source.

Geophones picked up a series of “spikes” that occurred over a period of about five minutes. The noises have not been heard again since.

“We're not sure what that means," said Mine Safety and Health Administration director Richard Stickler during a late press conference Wednesday.

“That has created a very small amount of hope and optimism. And that's my reaction."

Mine co-owner Robert Murray added that while the sounds may have come from somewhere else, such as rock movement or the activity of an animal, they still provide hope.

“Those sounds could have been anything, and we've said that from the beginning," he noted during a conference Thursday afternoon Utah time.

Plans for drilling have been changed with this news, and drilling for the fourth vertical borehole into the East Mountain over the Crandall Canyon mine has already begun, confirmed various media outlets onsite.

It will probably be about two days before this shaft reaches the new area, being targets 1586 feet below the surface, Murray noted.

MSHA provided progress reports on the other three boreholes, noting that air samples continue to be drawn from the first and compressed air is being forced down the second.

“Recent air samples drawn from borehole 3 indicate 16.8 percent oxygen and 21 parts per million carbon monoxide,” the agency added.

Underground efforts are also coming along aside from one stoppage Wednesday, according to the agency.

“Seismic activity occurred Wednesday night around 11pm EDT, forcing rescue workers to temporarily suspend efforts,” MSHA said.

“The continuous miner has advanced approximately 826 feet into the mine. Sixty-five people are currently underground, including seven MSHA employees.”

Murray said Thursday afternoon that another delay was suffered earlier following another shaking incident.

“I'm so sorry, ladies and gentlemen, to tell these families that we're not making better progress underground," he said.

“And it's strictly due to the fact that the mountain is still alive and the mountain is not allowing us to advance as rapidly as we would like to.

“The seismic activity underground has just been relentless … the mountain is still moving and we cannot endanger the rescue workers as we drive toward these trapped miners.”

Stickler concurred: “We’re doing the very best we can as fast as we can. You couldn't get another person into that working area.”

Crandall Canyon, part of the Genwal complex half owned by Murray Energy division Utah American – with partnering owner Intermountain Power – is in Huntington, Utah, about 150 miles south of Salt Lake City.

The 1.1 million ton per annum pillar operation employs 67 workers, 64 of whom call underground home and work in seven-man miner section crews on 12-hour shifts to keep production going 24/7.