A sleeping volcano's lethal breath
Just 20 kilometers southeast of Rome lies Lake Albano, a deep body of water filling the crater of a quiescent volcano. The lake is the deepest of its kind in Italy, plunging to about 170 meters. It occupies a complex maar—a crater formed by explosive eruptions when magma meets groundwater—within the larger Alban Hills Volcanic Complex. While the last major eruption occurred around 36,000 years ago, the volcano is not extinct. Deep beneath the lake bed, a magma chamber continues to release gases, primarily carbon dioxide (CO2).
This CO2 seeps upwards and dissolves under high pressure into the cold, dense bottom waters of the lake. Lake Albano is a meromictic lake, meaning its water layers do not easily mix, which allows the dissolved gas to accumulate to potentially dangerous concentrations. This geological situation creates the conditions for a rare and deadly natural disaster known as a limnic eruption.
The Lake Nyos precedent
The danger of such gas-charged lakes became terrifyingly clear on August 21, 1986, at Lake Nyos in Cameroon. A sudden overturn of the lake's water layers released an estimated 100,000–300,000 tons of CO2. The resulting gas cloud, being heavier than air, flowed down surrounding valleys at up to 50 kilometers per hour, silently asphyxiating 1,746 people and over 3,500 livestock in villages as far as 25 kilometers away.
Scientists recognized that Lake Albano presented a similar risk. Concerns intensified in the late 1980s and early 1990s. An earthquake swarm from 1989-1990 was followed by a significant injection of CO2 into the lake's deepest layers. Ground uplift in the Alban Hills, totaling about 30 centimeters between the 1950s and 1990s, also indicated that magma was accumulating and increasing pressure below. Monitoring by Italy's National Institute of Geophysics and Vulcanology (INGV) confirmed the rising threat, prompting action to prevent a Nyos-type disaster in a much more densely populated area.
Engineering a solution
Unlike the passive degassing pipes later installed at Lake Nyos, the situation at Lake Albano stabilized without direct intervention to remove the gas. Monitoring data from the decades following the 1990s seismic event showed that the CO2 content in the lake gradually decreased, reaching a more stable state. The lake's natural dynamics, including partial winter overturns, appear to release accumulated gas periodically, an important difference from the permanently stratified Lake Nyos.
Continuous monitoring remains active. Scientists from INGV periodically measure the vertical profiles of the water's chemical and physical parameters, including dissolved gas concentrations. This ongoing monitoring is essential, as a future large earthquake or magmatic event could inject another massive pulse of CO2 into the lake, overwhelming its natural degassing capacity and renewing the hazard. The stability of the lake is a delicate balance, and understanding its carbon dioxide budget is essential for protecting the millions of people living nearby.