I never heard of lake overturn until yesterday:

In 1986, before research into the Monoun disaster was made public, it all happened again. The tragedy of Lake Nyos, also in Cameroon, made headlines around the world when almost 1,800 people sleeping in houses around the lake suffocated in their sleep. The team of scientists that went to investigate concluded that carbon dioxide, trapped at the bottom of the lake, had suddenly risen to the surface, killing everything within 25km. They called their theory lake overturn. …

The Nyos disaster promoted a survey of deep lakes in Africa and Indonesia to see where else lake overturn could happen. All seem to be safe, except one – Lake Kivu, in Rwanda. Lake Kivu is one of the largest and deepest lakes in Africa and two million people live around its shore.

Kivu is also filled with methane. It could potentially cause large explosions above the lake that would endanger all two million people.  The show was on The Learning Channel, but the full transcript is available on the BBC website.