A massive crop circle has been discovered in Ireland, but it wasn't aliens who made it.

Five thousand years ago - before Stonehenge and the Great Pyramid of Giza were erected - ancient Irish stuck timber posts in the ground in a circle, and the effects can still be seen now.

Archaeologists have recently been excavating Neolithic structures at Newgrange, about 40km north of Dublin. A drone sent up by a local historian and photographer earlier this month returned with images of a never-before-seen circle in a field right next door to where they were digging.

"What the f**k is that?" shouted photographer Anthony Murphy, who first saw the site in his own drone footage, NPR reported last week.

"When we saw this, we knew straight away this had never been seen or recorded before."

Unlike than the flattened crop circles UFO pranksters like to make, this circle cannot be seen from the ground. Instead, the plants that make up the circle are of a slightly darker shade of green than the rest.

"Over time, when the monument fell out of use, the wood all rots away and the holes kind of fill up with organic material," said Mr Murphy. "But they leave a sort of a fingerprint, or a footprint."

The recent heatwave and drought across the UK and Ireland has starved the countryside of water, but the material that filled the holes where the posts used to be "has a very small advantage in terms of additional water and it's very slightly healthier".

In normal conditions the difference is unnoticeable, but the drought has brought the ancient monument into clear view.

It's a "rare opportunity to uncover further secrets held in our landscape", said Irish culture minister Josepha Madigan.

"With further research we know these fascinating finds will add greatly to our knowledge of the wonderful Brú na Bóinne World Heritage landscape."

The United Nations Educational, Scientific and Cultural Organization calls the area "Europe's largest and most important concentration of prehistoric megalithic art".

The Neolithic period began with the development of farming, and ended when metal tools became widespread.

It's believed Stonehenge was built a few hundred years later, around the same time as the Great Pyramid in Egypt.

Newshub.