As said, limnic eruptions are so singular that there are only two recorded incidents in history. While these explosions are related to volcanic explosions, they don't necessarily coincide with one. What essentially happens is that carbon dioxide (CO2) builds up in the bottom of the lake "from underlying magma reservoirs."
According to the Journal of Limnology, an external trigger, "possibly related to uncommon weather conditions, volcanic-seismic activity, or landslides" causes an invisible, but enormous, cloud of CO2. Since CO2 is heavier than air, the gas cloud travels through low-lying areas, causing those in its path to suffocate. However, the exact cause of limnic eruptions still isn't entirely understood. What's more, there's no warning sound other than a loud boom that occurs when the gas erupts from the lake. Some survivors have also reported a "strange smell," which is thought to be related to volcanic fumes such as sulfur.
The first recorded limnic eruption occurred on August 15, 1984, at Lake Monoun in Cameroon. Tragically, 37 people were killed along with countless animals. Around the eastern edge of the lake, the vegetation was flattened for over 100 yards, which meant that the eruption likely caused a 30-foot tsunami. Victims were both burned and asphyxiated, suggesting that there may have been some hydrochloric acid mixed in with the CO2. Hanging upwards of 10 feet off the ground, the gas cloud reportedly took almost four hours to fully dissipate.
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