  Mobile gravimetry is important in metrology, navigation, geodesy, and
geophysics. Atomic gravimeters could be among the most accurate mobile
gravimeters, but are currently constrained by being complex and fragile. Here,
we demonstrate a mobile atomic gravimeter, measuring tidal gravity variations
in the laboratory as well as surveying gravity in the field. The tidal gravity
measurements achieve a sensitivity of 37 $\mu$Gal/$\sqrt{\rm Hz}$ and a
long-term stability of better than 2 $\mu$Gal, revealing ocean tidal loading
effects and recording several distant earthquakes. We survey gravity in the
Berkeley Hills with an accuracy of around 0.04 mGal and determine the density
of the subsurface rocks from the vertical gravity gradient. With simplicity and
sensitivity, our instrument paves the way for bringing atomic gravimeters to
field applications.
