The Mount St Helens volcano in the United States has crooked plumbing. Rather than sitting above the magma source that feeds it, the mountain is off to one side.
A major eruption in 1980 saw 57 people killed by the volcano, which is in Washington state. Steven Hansen at the University of New Mexico in Albuquerque and his colleagues set off explosives around Mount St Helens and measured the way the seismic waves travelled through the ground. They confirmed that the rock beneath the mountain is too cold to create magma. Instead, the molten rock that feeds the volcano seems to come from off to the east.
The sideways plumbing helps to explain why the mountain remains active despite lying to the west of most neighbouring volcanoes.