  The world's largest cosmic-ray detector is nearing completion in the remote
Pampas of Argentina. This instrument measures extensive air-showers with
energies from ${10^{18}-10^{20}}$ eV and beyond. A surface detector array of
area 3000 ${km^2}$ records the lateral distribution of charged particles at
ground level. A fluorescence detector overlooking the surface detector records
the longitudinal light profiles of showers in the atmosphere to make a
calorimetric energy measurement. A ``test beam'' for the fluorescence detector
is generated by a calibrated laser near the array center. This talk will focus
on detector characterizations essential to the first science results that have
been reported from the observatory. Plans to construct a larger instrument in
the northern hemisphere will also be outlined.
