The Lake Nyos Disaster of 21 August 1986, When a Lake Exhaled and Killed 1,746 People in Their Sleep

Lake Nyos sits in the crater of an old volcano in Cameroon’s Northwest Region — deep, still, and picturesque. Beneath it, magma slowly leaks carbon dioxide into the groundwater, and in Nyos’s case the lake’s layered, rarely-mixing waters had been quietly absorbing that gas for centuries. By 1986 the bottom waters held CO2 under pressure like an unopened bottle of soda water the size of a valley.

Local oral tradition spoke of lakes that killed, of spirits that rose from the water; villagers in the region traditionally avoided building on some lakeshores. Science had a hint too: two years earlier, in 1984, nearby Lake Monoun had released gas that killed 37 people. The report was so strange it was widely doubted.

The night of 21 August 1986

Around 21:00, something — a landslide, unusually cool weather mixing the layers, no one knows precisely — disturbed the lake’s stratification. The deep water rose, the pressure came off, and the dissolved gas erupted out of solution: a limnic eruption. Witnesses on high ground described a rumble, a fountain jetting perhaps 100 metres from the surface, and a white mist filling the crater.

An estimated 1.6 million tonnes of CO2 poured out within minutes. Carbon dioxide is half again as dense as air; the cloud hugged the ground and flowed downhill at up to 50 km/h into the valleys towards the villages of Nyos, Kam, Cha, and Subum, displacing the breathable air as it went.

1,746 people died, the great majority in their sleep, along with some 3,500 cattle and countless birds and insects — a zone of total silence up to 25 kilometres from the lake. Survivors woke hours later, weak and confused, to find their families dead beside them, unmarked. Some 4,000 survivors suffered gas injuries and lesions from the accompanying acidic damp.

Defusing the lake

The event forced science to accept limnic eruptions as real, with exactly two confirmed instances — Monoun 1984 and Nyos 1986 — both in Cameroon. Since 2001, engineers have run degassing pipes from the lake bottom: self-powered fountains that vent the deep water’s gas continuously and harmlessly, gradually lowering the pressure. Two more pipes were added in 2011.

A third lake carries the same load at vastly greater scale: Lake Kivu, on the Rwanda-DR Congo border, holds dissolved CO2 and methane in its depths with over two million people living on its shores. Rwanda now extracts the methane for power generation — mining the hazard before it can erupt.