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.
By looking at the things that upset clocks, it's possible to map factors like magnetic fields and gravity variation. "Environmental conditions can make the ticking rate vary slightly," says O'Brian.
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.
How Super-Precise Atomic Clocks Will Change the World in a Decade