A meromictic lake has layers of water which do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters. This mixing can be driven by wind, which creates waves and turbulence at the lake's surface, but wind is only effective at times of the year when the lake's deep waters are not much colder than its surface waters. In a "monomictic" lake, this mixing occurs once a year; in "dimictic" lakes, the mixing occurs twice a year (typically Spring and Autumn), and in "polymictic" lakes the mixing occurs several times a year. In meromictic lakes, the layers of the lake water remain unmixed for years, decades, or centuries.
Occasionally carbon dioxide (CO2) or other dissolved gasses can build up relatively undisturbed in the lower layers of a meromictic lake. When the stratification is disturbed, as could happen due to an earthquake, a limnic eruption may result. _Wikipedia