MULLAN (AP) - Rescuers have blasted through 163 feet of solid rock in an effort to find an Idaho silver miner who has been missing since a cave-in last week, Hecla Mining Co. said Friday.

But the company says the most difficult work is still to come, as they advance into possibly more treacherous conditions.

Workers still have about 57 feet to go before they reach a void inside a collapsed tunnel where they hope Larry Marek, 53, will be found more than a mile deep in the Lucky Friday Mine.

"Crews have been progressing safely and quickly," Hecla said in a press release just before 6 p.m. on Friday. "However, as the crew advances closer (to where Marek may be), the work involved will become much more difficult, time consuming with new materials, equipment and supplies in order to advance the tunnel into potentially unstable ground."

Hecla has decided not to start digging on a second rescue tunnel, to prevent interference with stability monitoring on the current excavation effort.

The collapse happened last Friday. Rescuers have had no contact with Marek since.

Hecla Mining said workers also continue to pump fresh air and water into separate bore holes that reach into the clear area behind the cave-in. Small cameras sent into that so-called "void" have yet to show any images of Marek.

"We have secured additional cameras from external sources which will help us see inside the open areas," the company said, adding it's also testing air quality in the area of the collapse.

Marek and his brother, Mike, had just finished watering down blasted-out rock and ore in an area called Stope 15, which has been mined for 14 years, Hecla said. The ceiling collapsed about 75 feet from the rock face of the 6,150-foot deep tunnel, the company said. Mike Marek, who was working at the opposite end of the collapse from his brother, escaped unharmed.

Marek has been trapped with little food and water, likely in the dark, in temperatures well over a 100 degrees.

Rescue workers are drilling and blasting their way through rock in an effort to reach Marek's last known location.

No cause has been established for the cave-in. The mine has shut down production to concentrate on the rescue effort.

The Marek family has not spoken with reporters since the cave-in.

Hecla is the largest silver producer in the nation, from the Lucky Friday and the Greens Creek mine in Alaska.