On the night of August 21, 1986 in Cameroon, Lake Nyos suddenly released a huge cloud of carbon dioxide gas that claimed nearly eighteen hundred lives. Without a volcanic eruption, this horrifying tragedy revealed an extremely devastating natural hazard.

Referring to natural lakes, people often think of peaceful scenery with rolling ripples, abundant aquatic resources or peaceful stops for tourists.

But at Lake Nyos in Cameroon, a deadly secret once hidden under the calm water has turned this place into the scene of one of the most mysterious and horrifying tragedies in human history. There were no volcanic eruptions, no hot lava flows, and no seismic warning signals recorded before the disaster struck.

On the night of August 21, 1986, the tranquility of rural Cameroon was torn apart by an invisible murderer. Nearly 1,800 people and thousands of livestock died in just a few short hours, many people did not even have time to get out of bed.

The culprit of this tragedy was determined to be a giant cloud of carbon dioxide rising from the bottom of the lake. Sharing in an interview, Mr. Isaac Konfor Njilah, a geologist at Cameroon's University of Yaounde, affirmed that this event had left all experts completely stunned because there were no warning signs.

Nestled in a crater in the Oku volcanic region, Lake Nyos possesses a very unique geological structure. Although the surrounding surface volcanic system has long been dormant, pockets of magma deep underground still continuously absorb carbon dioxide through cracks in the Earth's crust to mix with underground water.

When it flows into Lake Nyos - where the depth is up to about 208 meters - the enormous water pressure at the bottom holds this huge amount of gas in a dissolved state. At the same time, strict water stratification has completely prevented natural convection between the gas-rich deep layer and the atmospheric surface. This situation causes Lake Nyos to accumulate gas for many years, like a giant bottle of carbonated water being compressed at the bottom.

That fragile balance finally collapsed that fateful night. Scientists call this phenomenon limnic eruption - a state in which a water solution containing gas becomes oversaturated and suddenly releases dissolved gas in the blink of an eye.

According to a study published in 1992 based on eyewitness accounts, the lake bed had shown unusual signs since August 20 with hot water rising. On August 21, small explosions, intermittent water columns and a rising stream of white smoke were recorded before a violent upheaval occurred at around 10 p.m.

When the bottom water layer begins to reflux, the sudden drop in pressure causes air bubbles to explode, reducing the density of the water and causing a chain reaction that pushes the entire mass of deep water to the surface.

A study conducted in 1987 based on chemical, geological and medical analysis samples confirmed that about 1.6 million tons of carbon dioxide from magma accumulated at the bottom of the lake were ejected directly into the air in a short time without the participation of direct volcanic activity.

Because it is heavier than normal air, the dense toxic gas cloud does not rise high but crawls along the hillsides, overflows through valleys and covers residential areas within a radius of up to 25 km. Too high a carbon dioxide concentration quickly displaces oxygen and directly affects the nervous system, causing the victim to lose consciousness and have immediate respiratory failure.

The final number of casualties recorded was 1,746 people and countless livestock and pets. The horrifying memories are still imprinted through the accounts of lucky witnesses who survived. Tsang Emelda Njah, then 5 years old, told DW that the ground shook so violently that her family thought the house was about to collapse so they rushed out.

Meanwhile, another victim, Mr. Ndong Frederick Bah, bitterly shared that his entire family of 9 went to sleep that night, but the next morning he was the only one left alive. Previously, in August 1984, a similar tragedy claimed 37 lives at neighboring Monoun Lake, but scientists at that time had not yet been able to fully decipher the dangerous mechanism of this phenomenon.

To prevent the risk of disaster recurring due to carbon dioxide gas continuously seeping into the lake bed, engineers have developed a streamlined but highly effective technical solution. In 2001, the first exhaust pipe was dropped deep into the bottom of Lake Nyos, followed by additional pipes in 2011, similar to the system applied at Lake Monoun.

This mechanism takes advantage of the pressure drop when the water flow rises in the tube, causing gas bubbles to automatically form and push the flow continuously to the surface to slowly release toxic gases into the atmosphere without using forced pumps.

If a 2005 study warned that the gas release rate was still too slow, only reducing the amount of toxic gases by 12 to 14 percent, then in 2020, a new scientific report confirmed that the program had successfully completed its set goals. Most of the long-standing carbon dioxide accumulation has been safely released, and the maintenance of a single pipeline is now sufficient to fully balance the rate of natural gas accumulation from the ground.

After four decades, Lake Nyos is much safer today and it is unlikely that a disaster of similar scale will recur. However, the monitoring system still has to operate continuously to monitor the amount of gas leaks.

In addition, social wounds have not yet healed as the Anglophone conflict in Cameroon continues to hinder people from resettling and limit scientific research activities.

The invisible obsession from the deep water still weighs heavily on the minds of survivors as Ms. Njah confided, that even though she was a child born in Nyos, she still felt scared every time she had to step near the lake shore.

This tragedy will forever be a valuable lesson that shows that the most severe natural disasters sometimes do not originate from earthquakes or roaring volcanoes, but from invisible dangers that accumulate quietly over the years.