00:00This lake released a deadly cloud that killed more than 1,700 people.
00:04Lake Neos fills a volcanic crater in northwestern Cameroon.
00:08More than 200 meters below the surface, the water near the bottom remained largely isolated.
00:14Carbon dioxide from deep within the earth accumulated in that bottom water.
00:18Under pressure, it stayed dissolved instead of bubbling to the surface.
00:22On the evening of August 21, 1986, gas-rich deep water began moving upward.
00:29Exactly what triggered that movement remains uncertain.
00:33As the water rose, pressure fell, and carbon dioxide formed bubbles.
00:37The bubbles pulled more gas-rich water upward, creating a runaway chain reaction.
00:42The gas was invisible, odorless and denser than air.
00:46It crossed the crater rim and flowed downhill through the surrounding valleys.
00:50People exposed to the cloud rapidly lost consciousness as oxygen was displaced and carbon dioxide levels rose.
00:58More than 1,700 people and thousands of animals died.
01:03Low-lying areas suffered the worst effects, while higher ground was less affected.
01:08The pattern showed that the dense gas had hugged the valleys.
01:11Even after the disaster, carbon dioxide continued entering the lake.
01:16Engineers installed deep pipes that release it gradually through self-sustaining fountains.
01:21Lake Neos is safer today, but carbon dioxide still enters its depths.
01:26Keeping it safe still depends on continued gas removal and monitoring.
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