A: Yes. Nature does not ensure safety.
Carbon dioxide produced by volcanoes leaks into the water and accumulates in high concentrations at the bottom of three deep lakes in western Africa. Natural disasters have occured at two of them.
In 1986, some disturbance caused the carbon dioxide from the bottom of Lake Nyos in Cameroon to rise to the surface. The disturbance is still unknown, but scientists now believe it could have been a landslide, localized heating, wave action, or a process called limnetic eruption in which carbon dioxide bubbles spontaneously from saturated water. As the carbon dioxide rose, the pressure decreased and the carbon dioxide formed gas bubbles. A cloud of carbon dioxide escaped from the lake. The high concentrations killed livestock and people.
In 1984, a similar event occured at another deep lake, Lake Monoun in Cameroon, but with much smaller loss of life. A third deep lake on the border of Rwanda and the Democratic Republic of Congo, called Lake Kivu, also accumulates gases including carbon dioxide at depth, but has remained stable.
Since the time of the disasters, scientists from France and Cameroon have collaborated to install degassing devices in Lakes Nyos and Monoun to help prevent carbon dioxide from accumulating in the deep water.
Image from http://www.geo.arizona.edu