A Roman coin was found resting just three inches down in the ground by a man using a metal detector.
Policeman Richard Malin found the ancient coin on an unmapped track near his home in Ashford in Kent.
He said when he took it out of the earth it shone and was not tarnished like other coins he has found.
The coin has been valued at about £7,000 and is due to be auctioned at Sotheby's in November. Mr Malin will share the money with the landowner.
Mr Malin, originally from New Zealand, said he started searching on the track after realising it did not appear on any maps.
He said "I found a couple of coins dating from 1790 and then I found the Roman coin.
"I got it out of the ground - it was about 3in deep - it was not deep at all and it shone like it had just been minted yesterday.
"I did not have to clean it, I just brushed the dirt off it."
He said other coins he has found have been tarnished but the Roman one was so clean you could clearly see the writing and portrait.
The British Museum told Mr Malin that the coin probably dated back to just after the Roman rule had finished and may have been dropped by a trader heading to London.