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A spring scale in a geologist's backpack responds sensitively to every mountain and valley beneath their feet. But springs get tired, and their readings drift — the instrument needs frequent recalibration against a reference. An atomic gravimeter measures gravity flawlessly, but it's the size of a refrigerator and doesn't like being jostled.

In a tropical forest, over an area of 24 square kilometers, they set up an experiment: two lightweight spring gravimeters were carried along survey routes, while an atomic one sat at base camp as the reference. The spring instruments returned to it for recalibration, and their drift, the slow creep of readings, was calculated and subtracted. In seven days, they produced a gravity map with microgal accuracy: that's one millionth of the normal gravitational force with which Earth pulls an apple.

Usually, such accuracy comes only from lab instruments. Here, it was achieved in the field, amid heat, humidity, and shaking. The hybrid scheme, an atomic reference plus lightweight workhorses, could become the standard for surveying large areas in hard-to-reach places.