I had never heard of this, though it happened in my lifetime. Perhaps the news cycle was all about the reactor explosion and fire in a place that is often called “Chernobyl”, which happened earlier that year.
On 21 August 1986, a limnic eruption at Lake Nyos in northwestern Cameroon killed 1,746 people and 3,500 livestock.
The eruption triggered the sudden release of about 100,000–300,000 tons (1.6 million tons, according to some sources) of carbon dioxide (CO2). The gas cloud initially rose at nearly 100 kilometres per hour (62 mph; 28 m/s) and then, being heavier than air, descended onto nearby villages, displacing all the air and suffocating people and livestock within 25 kilometres (16 mi) of the lake.
A degassing system has since been installed at the lake, with the aim of reducing the concentration of CO2 in the waters and therefore the risk of further eruptions.
I remember reading about massive CO2 gas clouds, caused by burn-off methane, being a thing in the Permian.
If I understand correctly we aren’t going to have to worry about that, exactly, because humans are busy extracting and burning all those bubbles of gas, already. The CO2 clouds may be a result of other processes, like algae blooms, or stagnant water.
The one thing that sticks in my mind is “16 miles is a hell of a long way to run” especially when you’re out of breath. I imagine that the typical human reaction to something like this would be incomprehension until it was too late to do anything else.