Gas eruption in a lake situated in an old volcanic crater.
Gas seeps into the lake from the ground underneath, and under the high pressure on the lake bottom it saturates the water, leading to high concentrations of gases like CO2 and Methane. A disruption of the water, such as an earthquake, causes some of the gas to reach higher up in the lake, where the pressure is lower. The gas can then escape the water, forms bubbles and travels to the surface. This opens a gap for more gas to do the same, and a chain reaction occurs.
This eruption of gas can cause a decent-sized tsunami, and the cloud of gas that is left on the surface will travel along the ground, causing death by suffocation. If the gas involved is Methane it can also catch fire and cause an enormous explosion.
The limnic eruption of lake Nyos in August 1986 killed nearly 2000 people and thousands of animals, yet nobody takes the risk of the same thing happening in a much larger scale in the enormous lake Kivu seriously enough to spend money on preventing it. Wow.