The catastrophic event at Lake Nyos in August 1986 has resulted in a major scientific debate concerning its origin and how to prevent new casualties of this nature in future. We have tried, in the present paper, to interrelate the testimonies in time and place, and to interpret them in geological terms. It has been possible to draw a hazard map of the catastrophe and reconstruct the sequence of the events. It began on August 20, 1986 with minor upwelling of hot water. On August 21, 1986, a small explosion occurred in Lake Nyos followed in the evening by an intermittent jet of water topped by a white plume. At 10 p.m. a major detonation occurred in the lake and carbon dioxide invaded the low lying valleys, killing more than 1500 people and 6000 head of cattle. The all scientific community agree: gases have a magmatic origin. Two scenarios have been used to describe the catastrophe: the volcanic origin scenario: a gas jet splitted across the lake water, and the limnic origin scenario: gases were stored in the lake water and an internal or external phenomenon triggered the degassing. In the volcanic origin scenario prevention of degassing is impossible, any volcanic area can present this danger in, or outside lakes; prediction will proceed with usual methods. In the limnic scenario both prediction and prevention are possible: the only dangerous area are located around lakes containing dissolved gases prevention can be obtain by pumping and degassing the deep waters to the surface. In the present paper, testimonies have been used to test both hypotheses.