I already mentioned the project CeNSE -Central Nervous System for the Earth- by HP, http://www.hpl.hp.com/news/2009/oct-dec/cense.html , a vision of a Earth covered by billions of sensors transforming our relation with it.
Now HP is moving from theory to practice with a partnership with Shell to find petroleum
The idea is to use a vast network of sensors to analyse seismic waves in order to detect oil fields. In fact, software applications can use seismic waves as a radar to map underground areas.
The sensors are basically tiny MEMS accelerometers one thousand times more sensitive than the one contained in a Wii controller or in a car to activate the air bag.
Mapping of an area may involve the deployment of hundreds of thousands of sensors connected one another through a mesh wireless network and generating PB of data.
It will be interesting to see how the experience from this initiative will trigger others, as an example in agriculture. I consider this as a very first step in wiring the Earth. Also, it is unlikely that all those sensors will be retrieved once the underground mapping is complete. It is much more likely that they will be left on place. Will they form a network that can be used by others for different applications.