When a wall is slowly covered over by earth, the materials it's made from decay and become part of the soils around and above it, sometimes causing vegetation above and next to the wall to grow faster or slower. Satellite imagery helps archaeologists to pick up these subtle changes.
There's even an aircraft sensor system that sends down hundreds of thousands of pulses of light measured at different return rates. It allows you to literally strip away vegetation and see entire cities beneath the rain forest canopy. This is the unbelievable future of archaeology.