On August 17, 1958, a severe earthquake in western Montana caused a great landslide from a nearby mountain to fall into the Madison River upstream from Ennis, Montana, completely blocking the flow of the river.
An estimated 43 million cubic yards of rock and earth crashed from the mountainside into the valley, forming a dam 300 feet high and more than a half mile long.
The Madison River soon backed up behind the barrier, forming a lake seven miles long.
At first, engineers thought that the dam would wash away.
But by cutting a spillway channel through the rock and earth dam, they lowered the level of the lake 50 feet and shortened the length of the lake to about four miles, greatly reducing the water pressure on the natural dam.
A dam of this size would cost about $50 million and take months or even years to construct, but the earthquake accomplished the feat in a few seconds.
The lake behind the dam is today named, appropriately enough, Earthquake Lake.
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