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Volcanic gases escape from the magma. They percolate through the earth crust and into the atmosphere. Professional geologist, Charles Watson, investigates the risks at Horseshoe Lake close to Mammoth's main resort. There is a quiet, silent killer that's lurking about in the trees, along the lake's edge.

In the early 1990s, trees began to die in the area north of the lake. The cause, high concentrations of carbon dioxide. Scientists have discovered that around 100 tons of the gas escapes around Horseshoe Lake each day, raising the level of carbon dioxide in the soil up to 95 times normal levels. A simple test at the lakeside shows what this means in practice. Carbon dioxide is heavier than air, so what we've done is we've dug a hole and carbon dioxide is gonna fill that up and displace the oxygen out. And extinguish this butane lighter. Fire needs oxygen to burn. Starved of this, the flame dies. It won't even relight. High concentrations of carbon dioxide can kill a butane flame in seconds and it can have a deadly effect on humans.

The usual concentration of carbon dioxide in air is only 0. 035 percent. At a concentration level of around 1 percent, it causes dizziness, headaches and hallucinations. Above 17 percent, unconsciousness and death happens in a matter of seconds.

At Mammoth Lakes, wind usually disperses the carbon dioxide safely. But in 2006 investigators confirmed that a lethally high concentration of the gas killed 3 ski patrollers who fell into a snow cave on Mammoth Mountain and there was no volcanic eruption.

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