Italian police uncovered an illegal rubbish dump hidden in the remains of ancient Roman catacombs on Monday and sealed off the area while they investigate alleged environmental pollution.

Underground caverns and tunnels used as tombs since the second century B.C. had been filled with harmful waste over the years, creating an underground lake of acrid oil, Italian media reported.

The city government confirmed police had seized the site, which lies along the Appian Way, a preserved example of the ancient Roman street-building technique which laid smooth stones over a bed of gravel.

Italy is home to some of Europe’s largest landfill sites and has been fined millions of euros by the European Court of Justice for failing to clean up its illegal dumping grounds.

The waste management business has also provided fertile ground for organized crime in the country’s poorer south, most notoriously in the “Land of Fires” north of Naples, where rubbish has been dumped and burned, poisoning the environment.