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A rare rainbow wedges itself between dark clouds and Chile's Atacama Desert in this photo, captured by Armin Silber, an employee at the European Southern Observatory. The sun's angled rays highlight not only the rainbow, but also the Atacama Large Millimeter/Submillimeter Array's Operations Support Facility, which spreads out at an altitude of 9,500 feet (2.900 meters) above sea level.

The Operations Support Facility is the base camp for the ALMA Observatory, which is at an even higher altitude on the Chajnantor Plateau in the Chilean Andes. ALMA is the world's highest and most advanced radio telescope array.

New types of equipment are assembled and tested at the support facility before they're taken up to ALMA's 16,400-foot (5,000-meter) level, primarily to minimize the adverse health risks associated with strenuous work at high altitudes. The air is denser at lower altitudes — and chances are that the rainbows look better as well.

— Alan Boyle