"Developed by Siemens, in collaboration with the Zaragoza University Hostile Environment Technology Group (GTE), the TEDRA™ device uses rock as a transmission element, providing a much greater level of security, and therefore facilitating, among other things, exploration work and cave rescue operations.
The sort of wireless communication we are used to is based on the transmission of electromagnetic waves (more commonly known as “radio waves”). Although able to travel through empty space or the atmosphere, radio waves are, in general, incapable of penetrating even a few metres underground. It’s not surprising then, that wireless communication in underground environments has proven to be a real technological challenge.
At BARRABES we had the opportunity to attend the presentation of TEDRA™, alongside representatives from specialized underground and rescue groups. After witnessing on-the-spot tests we were convinced that this system would revolutionize security for cavers and underground workers alike. "
Full INFO: http://www.barrabes.com/tedra/home.asp