Galaxy S4 Sensor Array: What can we build with it?

The Galaxy S4 contains a massive array of sensors: Compass (Magnetometer), Proximity, Ambient light, Gyroscope, Accelerometer, Barometer, Thermostat, Humidistat, Infrared. It's basically a simple mobile weather station! So we can build amazing weather maps. What else?

Can you map the Earth's magnetic field with a network of ten million S4s (plus so many other phones that have a compass)? What about an Infrared background map? Does that make sense?

What are your ideas?