But don't panic just yet. Scientists say it's not likely to blow its top for another 1,000 years or so.
Colli Albani is a volcanic complex spanning a 9-mile semicircle of hills, located approximately 19 miles away from the city center of Rome. It last erupted 36,000 years ago, prompting geologists to classify it as extinct.
However, according to a new study published in Geophysical Research Letters, scientists have observed new steam vents, earthquakes, and a rise in ground level in the surrounding area, suggesting that a bubble of magma is forming beneath the surface. These observations, along with evidence from satellite data, suggest that the volcano is starting a "new eruptive cycle" - in other words, it is rumbling back to life.
Researchers found that the ground is inflating at a rate of about 0.08 inches (2 millimeters) every year in areas where steam vents are appearing. Over the last 200,000 years, they said, the ground has risen a total of 164 feet. For thousands of years, the magma bubble under the volcano was trapped by pieces of land that have recently begun sliding against each other. If they slide enough, an eruption becomes possible.
Other signs that the volcano is brewing include a swarm of earthquakes around Rome from 1991 to 1995, as well as a small fumarole (steam vent) that appeared in a road near the Fiumicino airport in 2013.
Records of its previous eruption suggests that under the right conditions, Colli Albani could produce an eruption as destructive as Mt. Vesuvius, which destroyed Pompeii in 79 AD. A potential future eruption of Colli Albani could generate massive, far-reaching clouds of smoke and ash, unleash lava that would roll down the slopes at fast speeds, and send rocks raining on nearby cities. Rome itself would only be severely affected if the wind blew in the right direction, but the city's suburbs (which reach all the way to the base of the volcano) would be devastated.
Thankfully, researchers say the volcano won't blow unexpectedly. Sufficient pressure still needs to build for about 1,000 years before an eruption is even possible. Then, even when the conditions are just right, the explosive eruption would likely be preceded by initial states of moderate activity.