The dam, which lies a mere 300km north of Cameroon’s capital city Yaounde and plugs volcanic waste on the north side of lake Nyos, is close to collapse according to Nisa Nurmohamed, one of two UN experts who visited the site on 25 September in response to local scientists and geologists’ fears.

A breach would release waters downhill through Cameroon and into neighbouring Nigeria, flooding approximately 10,000 people along with animals and crops.

Another consequence would be the release of carbon dioxide into the local area. The deadly gas is settled at the bottom of Lake Nyos under the pressure of water 200m deep, but if the lake was drained and the pressure lowered there would be a sudden eruption of toxic fumes.

In 1986, Lake Nyos suddenly released a deadly cloud of carbon dioxide into the air that travelled 27km through the local population, suffocating 1800 people in their sleep. Thousands of livestock were killed, and the once-fertile land was rendered barren. A reproduction of this tragedy, but on an even larger scale, is feared.

Estimations as to how pressing the repair work is vary, with some experts saying that a volcanic tremor could collapse the dam at any time, and others arguing that it will last longer. Nurohamed, of the Netherlands Ministry of Transport and Public Works, said she expects the dam to breach within 10 to 20 years, but disaster could theoretically occur in as soon as five.

Strengthening the dam in the estimated timeframe is not seen as a viable option, according to Nurohamed and fellow UN expert Olaf Van Duin in their report, which is backed by the UN’s Office for the Coordination of Humanitarian Affairs (OCHA).

Instead, the report recommends halving the height of the 40m dam and making Lake Nyos 20m shallower, which would reduce the pressure on the dam’s wall by an estimated 75% at a cost of roughly US$15M.

But this process would have to be a careful operation, with the water siphoned out slowly in a controlled manner and taken from the bottom of the river instead of the top in order to collect some of the carbon dioxide that sits on the bottom of the pool as a syrupy solution the along the way.

The UN experts’ report says that this process could be completed within a year and a half.