UK researchers have built a small device that measures tiny fluctuations in gravity, and could be used to monitor volcanoes or search for oil.
Such gravimeters already exist but compared to this postage stamp-sized gadget, they are bulky and pricy.
The new design is based on the little accelerometers found in smartphones.
To begin with, the team - from the University of Glasgow - tested it by measuring the Earth's tides over a period of several days.
Tidal forces, caused by the interacting pull of the Sun and Moon, not only drag the oceans up and down but slightly squash the Earth's diameter.
"It's not a very big squeeze, but it means that essentially Glasgow - or anywhere else on the Earth's crust - goes up and down by about 40cm over the course of 12-13 hours," said Richard Middlemiss, the PhD student who made the new instrument.
"That means that we get a change in gravitational acceleration - so that's what we've been able to measure."
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