Rubbish piled up outside the walls of Pompeii offers proof that the Romans recycled their waste, experts have said.
A team of archaeologists carrying out excavations on the site believe inhabitants collected rubbish in piles to make buildings for the ancient city.
The Romans realised that vast quantities of chunks of mortar and plaster and ceramic sherds, among other detritus, could be turned into valuable construction material for floors and walls, their research shows.
Prof Allison Emmerson is part of a large team that has been working extensively at Pompeii that have concluded that rubbish dumps outside the city were actually recycling plants - "staging grounds for cycles of use and reuse".
Almost the entire external wall on the city's northern side, among other sites, had waste piled up against it, in some areas up to several metres high. Most of the mounds were cleared in the mid-20th century, but are still being discovered today.
Prof Emmerson said these mounds were previously thought to have been the result of a dramatic earthquake that struck around 17 years before the volcanic eruption of AD79 that destroyed the city.
"As I was working outside Pompeii, I saw that the city extended into developed neighbourhoods outside the walls," she added. "So it didn't make sense to me that these suburbs were also being used as landfills."
Her research has shown that the Romans constructed waste-management sites "for refuse to be sorted, stored and gathered in large enough quantities to gain value, with easy access to roads and highways by which it might be moved in, out and around the city".
With fellow archaeologists Steven Ellis and Kevin Dicus, of the University of Cincinnati, she has studied how the ancient city was constructed: "We found that at least part of the city was built out of trash."
Scientific analysis found differences in dirt and soil, allowing the archaeologists to trace the movement of garbage. In some walls, broken wine jugs and mortar and plaster were found neatly concealed beneath a layer of plaster.
Prof Emmerson, who teaches classical studies at Tulane University, New Orleans, points out that modern society prefers its rubbish just to be taken away, out of sight, out of mind.
"What I've found in Pompeii is an entirely different priority, that waste be collected and sorted for recycling," she added.