A digger being used by workmen on a building site in Kent has unearthed 3,600 bronze Roman coins dating from AD330 to AD348.
The coins are being kept safe by Kent County Council
Archaeologists from Kent County Council (KCC) were called to the site in the Medway Valley after the digger arm overturned a pot containing the coins.
"The workmen saw all these coins come pouring out of the digger bucket," said Maidstone Museum's Laura McLean.
They will be transferred to the British Museum for cleaning and recording.
It is then hoped the hoard of coins can be put on display in Kent.
The county council's Andrew Richardson said: "In four years of dealing with all the treasure in Kent I have never dealt with anything on this scale.
"The remarkable thing is that someone has gathered these coins together and stashed them because they were no longer legal tender."
Dr Richardson said the coins featured the head of Roman Emperor Constantine and other powerful figures from the time.