  The Pierre Auger Observatory is planned to be upgraded so that the energy
spectrum of cosmic rays can be studied down to 0.1 EeV and the muon component
of showers can be determined. The former will lead to a spectrum measured by
one technique from 0.1 EeV to beyond 100 EeV while the latter will aid
identification of the primary particles. These enhancements consist of three
high elevation telescopes (HEAT) and an infilled area having both surface
detectors and underground muon counters (AMIGA). The surface array of the Auger
Observatory will be enhanced over a 23.5 km2 area by 85 detector pairs laid out
as a graded array of water-Cherenkov detectors and 30 m2 buried muon
scintillator counters. The spacings in the array will be 433 and 750 m. The
muon detectors will comprise highly segmented scintillators with optical fibres
ending on multi-anode phototubes. The AMIGA complex will be centred 6.0 km away
from the fluorescence detector installation at Coihueco and will be overlooked
by the HEAT telescopes. We describe the design features of the AMIGA
enhancement.
