The Milagro detector, or telescope, is a large artificial pond filled with distilled water and covered with a huge opaque tarp at an altitude of about 8,000 feet near the Los Alamos National Laboratory. It began collecting data in 1999. The water provides a dense medium where incoming particles are more likely to interact and create faint flashes of light. The light can be detected by photomultiplier tubes (pmt's), and computers can time the PMT signal to reconstruct the path of the incoming air shower.

There are 723 photomulitplier tubes arranged in 2 layers in the finished array. Because showers don't arrive from the same direction, they are detected at different tubes at different times. Clever geometry and a lot of computer time sorts out gamma-ray showers from proton showers and backtracks the gamma-rays to their source.

Milagro was built to detect the rare air showers from gamma-ray bursts. It sees mostly air showers created by protons from our sun, but computing filters most of that data out of the final reporting.

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