Low-frequency radio waves with the ability to penetrate rock are being credited in the dramatic rescue of an explorer who was trapped for nine hours in a cave in Alberta, Canada in August.

The solution uses QDX-M radios from QRP Labs.

Volunteers with the Alberta/British Columbia Cave Rescue, part of Search and Rescue Alberta, have been working with these digital radios for their light weight since 2003.

According to a press release from the rescue group, the volunteers who built the kit radios are caving enthusiasts with electrical engineering and amateur radio backgrounds. The release said that the radio waves are capable of carrying messages through more than 500 meters of solid rock.

Christian Stenner, Alberta coordinator for the rescue team, said the radio system works underground and does not need line-of-sight to the sky as cell phones and GPS do. The system was deployed in the August 15th rescue for the first time, enabling team members who entered the cave on Grotto Mountain to communicate via text messages to their colleagues. They gave progress reports and made requests for any supplies they needed.

The trapped person, who was not identified in any media reports, had become stuck in a tight point inside what is known as the Rat's Nest Cave. That section of the limestone cave is vertical and pitches downward.

It was midnight when the man was freed. Paramedics in the cave aided him through his exhaustion, risk of hypothermia and the compression of the body during the rescue process.

The rescue agency issued a statement afterward, praising the technology and the team.