We know a lot about the Roman Empire. Not only did famous Romans like Julius Caesar write about their own accomplishments and plaster their names and works on public buildings, historians also chronicled the rise and fall of the powerful civilization. But sometimes it's hard to know how the average person in the Empire was doing—while Caesar was off conquering Gaul, was the economy good? During the Year of the Four Emperors, when intrigue and infighting rocked the empire, was the government still minting money? As Katie Langin at Science reports, researchers recently found some insight locked in Greenland’s ice cap.

Besides the power of the legions, Rome’s might lay in its wealth, the cornerstone of which was a silver coin known as a denarius. Producing the silver needed to mint all those coins meant smelting silver ore, which produced a lot of lead pollution. Since the 1990s, researchers have realized that the lead pollution produced by smelters across the Empire drifted 2,800 miles and left traces in peat bogs in Scotland and the Faroe Islands and in ice cores from Greenland’s ice cap. But those layers were imprecise and could not give a year-by-year reading of how much silver was being produced.

Using new techniques, however, historians and ice core experts have been able to take a closer look at the ice, slowly melting the cores to get 12 lead measurements per year from the length of the Roman Empire, roughly 1100 B.C.E. to 800 C.E. The 1,900-year chronology mirrors many of the ups and downs of the Empire, as described by historians past and present. The research appears in The Proceedings of National Academy of Sciences.

Nicholas Wade at The New York Times reports that ice cores from Greenland are hard to get, and it can take years to carefully drill through all the ice to reach bedrock. Luckily, ice core expert Joseph R. McConnell—ironically of the Desert Research Institute in Reno—knew of a core that had to be abandoned and was able to convince the core’s drillers to let him analyze a section dating between 1235 B.C.E. and 1257 C.E.

Three foot sections of the core were slowly melted on a special heating pad. After the water was collected and analyzed, the core showed that lead smelting began around 900 B.C.E. when Phoenician began trading in the Western Mediterranean. Smelting hit its peak at the height of the Roman Empire, during the 1st century C.E. That contradicts some historians who argued that the Roman economy did best during the Republic, before emperors took over. “The nearly four-fold higher lead emissions during the first two centuries of the Roman Empire compared to the last decades of the Roman Republic indicate substantial economic growth under Imperial rule,” co-author and Oxford historian Andrew Wilson says in a press release.

In general, the smelting activity rises and falls with Rome’s civil wars and disease outbreaks. The levels finally drop to pre-Roman levels during the Antonine Plague of 165 to around 180 C.E. and they do not recover for another 500 years. It also plummets during the Plague of Cyprian in the 3rd century. “We found that lead pollution in Greenland very closely tracked known plagues, wars, social unrest and imperial expansions during European antiquity,” McConnell says in the release.

Of course the lead levels aren’t a perfect indicator of economic activity. For instance, during the reign of Nero, Rome dropped to using just 80 percent pure silver for its coinage, reducing the need for smelting silver and leading to a period where older coins were recycled. But the researchers account for these idiosyncrasies and naturally occurring lead from sources like volcanoes to create a timeline that just looks at the output from the smelters. “I wouldn’t say the lead pollution graph is a close reflection of GDP but it’s probably the best overall proxy for economic health we’ve got,” Wilson tells Wade.

The data also presents some mysteries. For instance, Langin reports that there are some lead-level spikes that do not correspond to any known events or economic upticks. Kevin Butcher, an ancient historian at the University of Warwick not involved in the study, tells her this raises the possibility that Rome occasionally overproduced and stockpiled coins.

Even though the lead levels show the economic power of the Roman Empire, they pale in comparison to the amount of lead pumped into the air during the Twentieth Century, when the ice cores reflect more than 50 times the level of lead being produced during Rome's heyday.