When you open Google Maps and look at fields in specific regions of France or Italy, it's not uncommon to see patterns like this -- thin bright lines tracing little rectangles. These are the foundations of roman villae, buried below the crops.
If you go on location, you'll see that the ground is full of fragments of bricks, cement, tiles -- and roman pottery.
Some of them are known sites. Many remain entirely unlisted. The large majority were never dug up at all.