Experts are scratching their heads about a series of rock markings discovered in north Northumberland.
Experts are calling on people to help them identify the markings
The markings - including a heart shape and one resembling a human footprint - are being investigated by archaeologists on rocks near Wooler.
The Newcastle University team, who were alerted to the carvings by a local farm-hand, have said they are baffled as to what they mean or who created them.
And fellow experts they have consulted are equally confused.
The markings found on isolated sandstone boulders, include a group of concave spherical shapes of around 20 centimetres in diameter.
Cup and ring
Another resembles an adult footprint, several deep scores and a heart-shaped marking.
People are now being encouraged to come forward with explanations and to help solve the mystery.
Dr Aron Mazel, research associate with the School of Historical Studies, has been investigating the markings with Northumberland and international rock art authority Stan Beckensall.
Despite having over 60 years experience of studying rock art between them, they have been unable to identify what they were.
Dr Mazel said: "They are not the cup and ring marks which we have been studying as part of the Northumberland prehistoric rock art project - they appear to be more recent than that.
One of the marks resembles a heart shape
"There have been people in the Northumberland area since the start of the Mesolithic period around 10,000 years ago, but I would think that these markings were made after cups and rings, probably during the last 3,000 years.
"Until we know more about these markings, it would be hard to pinpoint which era they belong to."
Mr Beckensall said: "As far as I know, these markings are unique and nothing like them exists anywhere else in Northumberland or in the British Isles.
"That's why we are keen to draw people's attention to them - seeing the pictures of the markings may prompt somebody to come forward with new information, perhaps relating to similar rock art samples they have viewed elsewhere."