Archaeologists have found the remains of what could be Britain's oldest surviving human brain.
The team, excavating a York University site, discovered a skull containing a yellow substance which scans showed to be shrunken, but brain-shaped.
Brains consist of fatty tissue which microbes in the soil would absorb, so neurologists believe the find could be some kind of fossilised brain.
The skull was found in an area first farmed more than 2,000 years ago.
More tests will now be done to establish what it is actually made of.
The team from York Archaeological Trust had been commissioned by the university to carry out an exploratory dig at Heslington East, where campus extension work is under way.
The skull was discovered in an area of extensive prehistoric farming landscape of fields, trackways and buildings dating back to at least 300 BC.
The archaeologists believe the skull, which was found on its own in a muddy pit, may have been a ritual offering.
It was taken to the University of York where CT scans were used to look at the skull's contents.
Philip Duffey, the consultant neurologist who carried out the scans, said the find was "amazing".
"It's exciting that scanning has shown structures which appear to be unequivocally of brain origin.
"I think that it will be very important to establish how these structures have survived, whether there are traces of biological material within them and, if not, what is their composition."
He added: "This could be the equivalent of a fossil. The brain itself would generally not survive. Fatty tissues would be feasted on by microbes.
"This isn't like the remains found in bogs; it doesn't have any skin on the skull or any tissue remains elsewhere.
"There is something unusual in the way the brain has been treated, or something that it's been exposed to that has preserved the shape of it."
Dr Sonia O'Connor, research fellow in archaeological sciences at the University of Bradford added: "The survival of brain remains where no other soft tissues are preserved is extremely rare.
"This brain is particularly exciting because it is very well preserved, even though it is the oldest recorded find of this type in the UK, and one of the earliest worldwide."
The find is the second major discovery during investigations at the site.
Earlier this year, a team from the university's department of archaeology unearthed a shallow grave containing the skeleton of a man believed to be one of Britain's earliest victims of tuberculosis.
Radiocarbon dating suggests that the man died in the fourth century, the late-Roman period.
The vice-chancellor of the University of York, Professor Brian Cantor, said: "The skull is another stunning discovery and its further study will provide us with incomparable insights into life in the Iron Age."
Specialists now hope to carry out further tests on the skull to establish how it has survived for so long, and perhaps more about the person whose brain it was.