Why is Old Faithful so faithful? Roughly every 90 minutes this spectacular geyser in the Yellowstone national park spouts a plume of boiling water 40 metres into the air. In Victorian times people used to put their washing into it, discovering that cotton shirts survived the high-pressure wash, but woollens were torn to shreds. It is one of the most predictable geysers in the world, and yet relatively little is known about its underground plumbing.
In 2016 scientists placed a dense network of seismometers around Old Faithful, and recorded her innermost gurglings over a two-day period (25 eruptions). Previously instruments placed in and around the geyser had provided a picture of the pipework down to around 20 metres’ depth, but the new data has probed right into the belly, to 80 metres’ depth. The results, which are published in Geophysical Research Letters, show how a tremor (most likely caused by hot steam condensing to liquid) drops rapidly to around 80 metres after each eruption, and then gradually travels up again to around 20 metres, before re-erupting. A bubble trap at around 20 metres’ depth appears to trigger the eruption. And because Old Faithful is far from other geysers there is little interference to this pattern, which explains why it stays so regular.
