Ants on the Melon
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From Wired: How Super-Precise Atomic Clocks Will Change the World in a Decade

At that level, clocks will be precise enough that they’ll have to correct for the relativistic effects of the shape of the earth, which changes every day in reaction to environmental factors. (Some of the research clocks already need to account for changes in the NIST building’s size on a hot day.) That’s where the work at the Time and Frequency Division begins to overlap with cosmology, astrophysics and space-time.

And

That means passing a precise clock over different landscapes yields different gravity offsets, which could be used to map the presence of oil, liquid magma or water underground. NIST, in short, is building the first dowsing rod that works.

On a moving ship, such a clock would change rate with the shape of the ocean floor, and even the density of the earth beneath. On a volcano, it would change with the moving and vibrating of magma within. Scientists using maps of these variations could differentiate salt and freshwater, and perhaps eventually predict eruptions, earthquakes or other natural events from the variations in gravity under the surface of the planet.