Lake Nyos in Cameroon is, at a glance, innocuous, and silent. It’s a crater lake, meaning that the water there poured in long after the incredibly explosive volcanic eruption that forged it went quiet four centuries or so ago. The lake is nothing particularly special, but the view from the top – elevated high above the surrounding viridian – is certainly spectacular.
This beauty betrays a grim history. On August 21, 1986, by that very same lake, one of the most bizarre natural disasters in history unfolded overnight, and 1,746 people, and thousands of cattle, living nearby died within moments. When they were found, there weren't any immediately obvious physical signs of damage or distress.
Some blamed it on the work of wrathful spirits; others, a chemical attack, carried out by insurrectionists or the government itself. It didn’t take much geological sleuthing to find the killer, though: One volcanologist sent to investigate was shocked when a water sample he’d put in a bottle caused the lid to pop off, indicating that the water was full of dissolved carbon dioxide.
A comprehensive review of the tragedy, published in Science in 1987, blamed the disaster on a sudden, violent release of colorless, odorless carbon dioxide, which – being denser than air – rolled down the slopes of the hilly topography and smothered all those in its wake, many as they slept.
Lake Nyos sits above an active volcanic system, one whose magmatic reservoirs contain plenty of carbon dioxide. This isn’t too unusual, as carbon dioxide is a common feature of plenty of magmatic systems. Near the surface, the magma experiences less confining pressure, so the carbon dioxide escapes from the partly molten mass, forms bubbles, gets into subterranean, geothermal fluids, and seeps into the surface.
If there’s a lake there, then the gas seeps into the sediment. Over time, this gas should leak out of the water and escape into the atmosphere without causing any sort of fuss whatsoever.
Lake Nyos, however, is different.