One phone-sized device can block cell phone signals in a 20 foot radius.

CNN: Mobiles used in high-tech terror

These eye-in-the-sky cell towers hover between 70, 000 and 100, 000-feet and provide coverage for a 200-mile radius on Earth- but the darn things only stay aloft for 24 to 48 hours before freefalling to the ground (or tree, or lake, or Grand Canyon).

ENGADGET: Cash-in on your geocaching skillz