Hot air balloon pilot Michael Wolf, of Reading Road, Wallingford, was training another pilot near North Stoke when they spotted several dark circles in a farmer’s field.
Crops had grown at different speeds because of ancient ditches hidden beneath the soil which once surrounded prehistoric burial mounds.
The site dates back 3,500 years but is hidden under fields and normally cannot be seen.
The 52-year-old said: “In twelve years of ballooning, I have never seen anything as clear as this. It was like looking down on a map.
“I have flown over this field before, and never seen any marks. The hot weather must have created absolutely perfect conditions for seeing the marks.”
He added: “There were seven or eight big circles. It was absolutely fantastic.”