Nestled at 4100 m on the slopes of Sierra Negra, an extinct volcano 200 km east of Mexico City, the High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) began waiting for prey on 21 August. Mexico is providing the site and about a quarter of the $13.8 million for construction, with the US agencies NSF and the Department of Energy splitting roughly two-to-one the rest of the binational project’s cost.

HAWC is capable of finding “the highest-energy photons ever detected by man, and higher than what can be produced by man,” says Brenda Dingus of Los Alamos National Laboratory. “The gamma rays we detect are produced where particle densities are high, so it’s one of the best ways to look for high-energy particle sources.” Sensitive to gamma rays in the 100-GeV to 100-TeV range, HAWC detects the Cherenkov radiation emitted when showers of charged particles that the gamma rays create in the...

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