I am a huge Time Team geek. Before every dig on Time Team, the team does a geophysical scan of the area in order to see anomalies in the soil. Sometimes they even go up in a helicopter-- Roman ruins are often visible as lines in fields, soil, or crops that can be seen from the air.
What archaeologists are doing in Egypt takes this type of scanning one step further. The are scanning with inferred images from satellites in order to discover tombs-- and possibly even pyramids-- under layers of silt. Listen to the story on NPR.