Julia Griffin:

Instead, the concentric rings of dots were crop marks, ghostly outlines of a civilization past emerging from the moisture-starved landscape.

Crop marks and their related phenomena, parch marks, allow drone operators and aerial archaeologist to see thousands of years of history beneath the dry soil without ever having to pick up a shovel.

Here's why they form. Over time, prehistoric ditches, moats and other dug-out features are filled in by subsequent generations. Today, those areas retain more water and dry spells than the surrounding earth and lead to taller and greener grasses and crops.

At the same time, thin layers of soil placed over old stone walls or building foundations cause vegetation above to dry out more quickly. The result is a variety of geometric figures visible only from above.