Mesh networks

Mesh networks are pretty cool. The basic idea is that instead of having a base station which everyone communicates with, every node in the network is a transmitter and a receiver. That way you can have hundreds of devices which communicate a short range.

These could be used as detection devices, if you are trying to see the interior of a building or a cave structure, you unload a few hundred and they each use heat/motion sensors, the mesh would give you a complete picture of any individuals in the building.

It is more interesting for cave structures because they could be mobile and randomly spread through the cave structure. As long as there is a chain of the devices that go up to the surface, the signals from the ones deep in the cave structure could be read outside of the cave structure.