I forgot to mention in my earlier post that just turning your cell phone off before attending an underground meeting is not good enough. You must leave it behind or put it in a double tin can. There is a back up battery in the phone that still allows it to transmit and receive a signal even when it is turned off. Accordingly, mics and cameras can also still be turned on even when the phone is turned off.

If you choose to use tin cans (and it would be good to have them at the meeting in case anyone forgets to leave their phone behind) you need to test their effectiveness by putting a cell phone that is turned on inside the cans and then calling it. If the phone still rings, try different cans, or try lining the one next to the phone with cardboard.