Far below the wide lava-stone streets of the ancient city of Pompeii, a vast network of water drainage systems once provided relief to the city during rainstorms, collecting excess rainwater and draining it out to sea. Now, a new exploration into the city’s underbelly has systematically cataloged the system and cleared it for functioning thousands of years after its construction.

The notoriously prosperous Roman city was destroyed following a catastrophic eruption of Mount Vesuvius in the 1^st century CE, blanketing its residents in volcanic ash and noxious gas that would kill nearly everyone in its path. Over 16,000 people were killed with many left frozen in time, leaving the city abandoned for about 2,000 years, according to History.

Since 2018, speleologists working with the Archaeological Park of Pompeii have explored 457 meters (1,500 feet) of underground passages in an attempt to study the city’s rainwater drainage system, said the park in a statement. A network of tunnels and canals that branch out from a pair of cisterns located below the town center is believed to have been built over the course of three phases: an initial Hellenistic phase in the 3^rd century BCE, a second phase in the late Republican age around the 1^st century BCE, and a third phase corresponding to the Augustan and Imperial age just before the city’s demise in 79 CE.

Experts cleaned deposits from the tunnels that had built up over the millennium in an effort to restore functionality to the system. They also identified potential issues and needed solutions to keep the drainage pipes functioning while respecting the archaeological site. Altogether, the work aims to broaden our understanding of the site, which is essential to monitoring and safeguarding its historic features. The discovery of the tunnels demonstrates that much still remains to be discovered about the ancient city.

“Furthermore, many gaps in knowledge from the past regarding certain aspects or areas of the ancient city are being filled, thanks to the collaboration of experts in various sectors, which allow us to gather ever more accurate data as a result of specialized skills which had never been employed in other periods of excavation or study,” said Park Director General Massimo Osanna.

The first phase of the project finished at the end of January. Experts say they will now work toward repurposing canals and cisterns to continue properly draining water.