DENVER, Colo. (AP) -- An international science group Tuesday chose Colorado as the site of a $50 million observatory to measure the cosmic rays that continually bombard the Earth.

The Pierre Auger Collaboration said the observatory -- a sprawling 40-mile-by-40-mile array of remotely monitored sensors _ will be built in the southeast corner of Colorado about 180 miles southeast of Denver.

It will be a counterpart to the collaboration's Southern Hemisphere observatory, in Argentina.

Jim Sites, associate dean for research at Colorado State University in Fort Collins, a member of the collaboration, said the observatory is designed to capture the extremely rare, highest-energy cosmic rays that pack so much power scientists cannot figure out what produces them.

"It's possible there's a very mundane answer, but my guess is there's going to be some light shed on something we have just been speculating on, or haven't even guessed at yet," he said.

Sites said only about five high-energy cosmic rays have ever been measured. Cosmic rays were first detected in 1912.

The Pierre Auger Collaboration -- a collection of universities and laboratories around the world _ hasn't begun lining up funding, he said.

The northern observatory, like the one in Argentina, will consist of 1,600 tanks filled with purified water. The water will catch a faint light emitted by secondary energy particles created when the rays hit the Earth's atmosphere, Sites said.

Electronic equipment powered by solar panels or batteries will measure the light and transmit the data by radio to a headquarters building at Lamar Community College in Lamar.

The collaboration chose the Colorado site because of its elevation, about 3,600 feet above sea level, and because it has easy access and room to expand, Sites said.