(CNN) -- Clusters of Roman skulls have been discovered deep below London's Liverpool Street by construction workers digging a new rail route through England's capital. Tunnelers working on the Crossrail project found about 20 skulls, deep beneath the 16th century Bedlam burial ground in the center of the city, Crossrail said in a statement. Read more: Rail excavation unearths suspected 'plague pit' The human skulls -- as well as fragments of Roman pottery -- were found in sediment of the historic river channel of the River Walbrook, a tributary of the River Thames, it said. Roman-era skulls had been found along the path of the River Thames throughout London's history, fueling speculation they were the heads of the victims of rebels fighting under Queen Boudicca against Roman occupation, lead archaeologist Jay Carver said.
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Construction workers digging a tunnel in London have found about 20 Roman-era skulls
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The workers were digging a tunnel for the Crossrail rail project spanning the city
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Archaeologists believe the skulls were washed down stream from an old burial ground
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More than 10,000 archaeological items have been found since the project began in 2009
Crossrail said tunnelers at Liverpool Street also discovered medieval wooden structures, which archaeologists believe may have formed part of the @placeholder cemetery walls.