According to Science Daily:
Scientists can't say for sure if the volatile mixture at the bottom of the lake will remain still for another 1,000 years or someday explode without warning. In a region prone to volcanic and seismic activity, the fragility of Lake Kivu is a serious matter. Compounding the precarious situation is the presence of approximately 2 million people, many of them refugees, living along the north end of the lake.
Vodacek likens the contained pressure in the lake to a bottle of carbonated soda or champagne. "In the lake, you have the carbon dioxide on the bottom and 300 meters of water on top of that, which is the cap," he says. "That's the pressure that holds it. The gas is dissolved in water."
When the cap is removed, bubbles form and rise to the surface. More bubbles form and create a column that drags the water and the gas up to the surface in a chain reaction.
"The question is, and what's really unknown, is how explosive is that?" Vodacek says.