  The Milagro Gamma Ray Observatory, located at an altitude of 8,600 feet in
the Jemez Mountains of New Mexico, is the world's first large-area water
Cherenkov detector capable of continuously monitoring the entire sky for
sources of TeV gamma rays. It is uniquely capable of searching for transient
sources of VHE gamma rays. The core of the detector is a 60m x 80m x 8m pond
instrumented with 723 PMTs deployed in two layers. This part of the detector is
complete and has operated continuously since Jan. 2000. Initial studies
including searches for gamma-ray sources are ongoing, and preliminary results
are available. The final stage of construction is under way. We are deploying
170 auxiliary "outrigger" water Cherenkov detectors in an area of 40,000
square-meters surrounding the pond, which will significantly enhance our
ability to reject background and more accurately reconstruct the gamma-ray
direction and energy. In addition, we are lowering the energy threshold of the
detector by using custom processing to enable real-time intelligent triggering.
The lower energy threshold will significantly increase our sensitivity to
gamma-ray sources, and in particular to sources of cosmological origin, such as
GRBs, where the higher energy gamma-rays have sizable attenuation due to the
interaction with the intergalactic infra-red light.
