Final answer:
Carbon Dioxide (CO₂) is the gas that causes instability in the bottom waters of Lake Nyos, leading to a deadly natural disaster in 1986, which was mitigated by installing a CO₂ vent.
Step-by-step explanation:
The gas that primarily causes instability in the bottom waters of Lake Nyos is Carbon Dioxide (CO₂). Lake Nyos is a rare type of deep volcanic lake that holds a large amount of dissolved carbon dioxide due to volcanic activity beneath it. This accumulation can lead to a dangerous phenomenon known as a limnic eruption, where the dissolved CO₂ suddenly outgases on a large scale.
In 1986, a devastating natural disaster occurred when Lake Nyos experienced such an event, releasing a massive cloud of CO₂ that suffocated over 1700 people in nearby villages. To prevent a recurrence, a CO₂ vent was installed to outgas the lake in a controlled manner, reducing the carbon dioxide saturation in the lake's deep water and curbing the risk of another sudden outburst.