It took an unusually brutal drought for signs of a 5,000-year-old monument to suddenly appear in an Irish field, as if they had been written into the landscape in invisible ink.

On Monday, Anthony Murphy, an author and photographer, sent a camera-enabled drone high above the Brú na Bóinne archaeological landscape, a Unesco World Heritage Site about 30 miles north of Dublin. He suspected that recent dry conditions might reveal evidence that a henge — a man-made enclosure from thousands of years ago thought to serve as a gathering place — had once been there.

What he and a friend saw in the images shocked him: a series of discolorations in the farmland, caused by differences in soil, spread about 150 meters wide in a perfectly circular pattern. He had flown the drone over the same field many times before and never saw a hint of what was now perfectly clear, he said.