In the early 1990s, researchers lowered a camera into Old Faithful's throat and, 7 metres down, saw a "choke-point" less than 10 centimetres wide. Below that was a cavern about the size of a small car. But the camera couldn't go deeper than 14 metres because of the violent churn of boiling fluids.
Now Jean Vandemeulebrouck of the University of Savoie in Le Bourget du Lac, France, and colleagues have analysed seismic data collected in the 1990s. Using a modern acoustic technique more commonly used to locate whales or submarines, they analysed the noise of the popping bubbles inside the geyser – captured in the seismic record – to reveal a 60-cubic-metre cavern 15 metres down, and off to the geyser's side.
This side cavity acts as a compressible reservoir, making the water in the smaller main vent bob up and down like a spring. The oscillations affect the water pressure and help the geyser to boil in the early stages of eruption.
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