Limnic eruptions are exceptionally rare for several reasons. First, a CO2 source must exist (regions with volcanic activity are most at risk). Second, the vast majority of lakes are holomictic (i.e., their layers mix regularly), preventing a buildup of dissolved gases. Only meromictic lakes do not mix and remain stratified, allowing CO2 to remain dissolved. It is estimated only one meromictic lake exists for every 1,000 holomictic lakes. Finally, a lake must be deep enough to have sufficient pressure to dissolve large amounts of CO2.