Lake Nyos in Cameroon is, at a glance, innocuous, and silent. It’s a crater lake, meaning that the water there poured in long after the incredibly explosive volcanic eruption that forged it went quiet four centuries or so ago. The lake is nothing particularly special, but the view from the top – elevated high above the surrounding viridian – is certainly spectacular.

This beauty betrays a grim history. On August 21, 1986, by that very same lake, one of the most bizarre natural disasters in history unfolded overnight, and 1,746 people, and thousands of cattle, living nearby died within moments. When they were found, there weren't any immediately obvious physical signs of damage or distress.

Some blamed it on the work of wrathful spirits; others, a chemical attack, carried out by insurrectionists or the government itself. It didn’t take much geological sleuthing to find the killer, though: One volcanologist sent to investigate was shocked when a water sample he’d put in a bottle caused the lid to pop off, indicating that the water was full of dissolved carbon dioxide.

A comprehensive review of the tragedy, published in Science in 1987, blamed the disaster on a sudden, violent release of colorless, odorless carbon dioxide, which – being denser than air – rolled down the slopes of the hilly topography and smothered all those in its wake, many as they slept.

Lake Nyos sits above an active volcanic system, one whose magmatic reservoirs contain plenty of carbon dioxide. This isn’t too unusual, as carbon dioxide is a common feature of plenty of magmatic systems. Near the surface, the magma experiences less confining pressure, so the carbon dioxide escapes from the partly molten mass, forms bubbles, gets into subterranean, geothermal fluids, and seeps into the surface.

If there’s a lake there, then the gas seeps into the sediment. Over time, this gas should leak out of the water and escape into the atmosphere without causing any sort of fuss whatsoever.

Lake Nyos, however, is different.

It has both a vast volume of water and incredibly still waters. The water pressure ensures that, it’s thought, the gas remains dissolved within the lake, and a strongly defined layer of CO₂-enriched fluid was allowed to accumulate for several centuries without being disturbed by any wave action. Each unit of water had five times the amount of carbon dioxide dissolved within it.

The trigger for what happened next is still up for debate. It was either caused by a landslide down into the layer or a sudden release of fresh carbon dioxide. The outcome, however, was assured either way: the natural soda can's cap was broken.

The stratification broke down, the gas exsolved extremely quickly, and was unleashed en masse – all 1.2 cubic kilometers (0.3 cubic miles) – of it. A colossal blast occurred, causing water to shoot skywards and outwards hundreds of meters.

It formed a mist with the water vapor lingering in the air. It blanketed villages as far as 25 kilometers (15.5 miles) away, and entire villages were wiped out. As noted by Atlas Obscura, plenty of those napping on the floor died in the same rooms as those who were standing, initially without them realizing anything had happened.

Nyos itself only had around 800 survivors when all was said and done, many of which suffered from vomiting and diarrhea, symptoms of CO₂ poisoning. Others had said they smelled gunpowder or had become incredibly warm all of a sudden – olfactory and thermoceptionary illusions brought on by the poisoning.

This so-called “limnic eruption” ended up becoming a mass asphyxiation event. The 1987 paper explained that the lake turned from blue to red, as the iron lingering at its base floated up to the top during the pandemonium.

As a result of this horrific incident, authorities have now installed a degassing system in the lake, as well as a solar-powered warning system. This ensures that carbon dioxide emerging from below can no longer accumulate, so a repeat of this natural disaster is unlikely.

Lake Nyos isn’t alone, though.

A similar limnic eruption took place in 1984 over at Lake Monoun, also within Cameroon territory, killing dozens. Like Nyos, Monoun also features a manual degassing system. Both take the form of rather aesthetically pleasing water fountains.

Incidentally, some villagers living near Lake Monoun claimed that evil spirits periodically escaped the lake in order to kill people. It’s not difficult to see why these freakish natural dangers inspire such scary-sounding tales.

Lake Nyos is certainly a cautionary tale, one that those residing near Lake Kivu certainly should take heed of.

Straddling the border between the Democratic Republic of Congo and Rwanda, it’s 2,000 times bigger than Lake Nyos and has 2 million people living by its shoreline. It’s also full of methane and carbon dioxide, trapped by a dense layer of salt – so the potential for another sudden effusion of lethal gas exists, just with far more people in the way.

Methane is also incredibly valuable as a source of cheap energy, so both nations have agreed to mine it as part of the Kivuwatt Biogas Project. They have to be extremely careful though: As noted by researchers, if the methane deposits are violently disturbed, it could rapidly degas into the atmosphere.

It’s recently been suggested that additional oil exploration should also take place within the lake. Experts have advised serious caution, noting that it’s not possible to eliminate the risk of a cataclysmic gas release. The last thing anyone wants is a doubled-down sequel to the horrific original two-parter in Cameroon.