Archaeologists working with London's Crossrail project have uncovered 20 skulls believed to be from the Roman period.
It is likely the bones were washed from a nearby burial site along one of London's "lost" rivers - the Walbrook.
In the last year archaeologists in London have also found about 10,000 Roman items at a nearby site.
These latest finds could give new insights into the lives of Roman people.
Near-intact pottery artefacts were also found which probably travelled along the same route as the skulls. Other bone fragments would not have been washed as easily down the river.
Paved over in the 15th Century, the Walbrook river divided the western and eastern parts of the city, its moist muddy walls providing exceptionally good conditions for artefacts to be preserved.
The discoveries were found about 3m below ground and underneath the the Bedlam cemetery, a burial ground where hundreds of skeletons have been unearthed.
Though they have yet to be forensically dated, Nicholas Elsden from the Museum of London Archaeology said they were likely to be from the 3rd to 4th Centuries AD, as that was when Romans buried their citizens as opposed to cremating them.
"It's relatively unusual to find so many concentrated [in one area] when you're not in a graveyard. We're 100 yards outside the Roman city walls."
Roman law required burial outside the city, explained Mr Elsden, which meant there were burial sites circled around the town.
"What we're looking at here is how the Romans viewed their dead. You wouldn't imagine modern burial grounds being allowed to wash out into a river," he told BBC News.
Don Walker, an osteologist also from the Museum of London Archaeology, said the skulls were probably buried in different environments, shown by their shades of brown and grey.
"Forensic studies show that when the body disintegrates near a watercourse, the skull travels furthest, either because it floats or it can roll along the base of the river.
"They were possibly buried in an area where there wasn't much land available. At the moment it looks as though they've collected together through natural processes."
From initial observations, Mr Walker said there was no evidence of any "foul play", but details about their sex and age would only emerge through further investigations.
He added that chemical markers on the teeth could reveal where these people came from and what sorts of food they ate.
Archaeologists believe that the Crossrail Project will lead to further discoveries hidden beneath the streets of London and say it could transform our understanding of Roman London.
Crossrail currently operates over 40 worksites and archaeological investigations are carried out at each site ahead of main construction works to build the central stations.
The project will connect 37 stations from Heathrow Airport and Maidenhead in the west, through central London and out to Abbey Wood and Shenfield in the east and is due to be completed in 2018.