While satellites and surveillance aircraft can identify areas, itís up to ground forces to confirm whether or not the suspicions stand up using not only old-fashioned intelligence gathering but also technology.
The latest equipment used by the US is based on well-known scientific advances in infra-red, laser and "spectroscopic" sensors.
These can create a picture of the world invisible to the human eye. What we do not know is exactly how far US military scientists have taken the technology and the computing power required to process the data.
Click on the numbers on the pictures opposite to find out more