Breathe. Breathe. I repeated these words to myself like a mantra. At 18,400 feet, my body was craving oxygen, and I had to concentrate on pulling enough air into my lungs. I was on the summit of Cerro Toco, a stratovolcano overlooking Chile’s Chajnantor Plateau, now home to the Atacama Large Millimeter/submillimeter Array, one of the world’s premier radio telescopes. Between the thin atmosphere and the barren red terrain of the mountain, it felt like I was on Mars. My colleagues and I were testing the atmospheric conditions on Cerro Toco. If they were good enough, they might justify taking on the technical challenges of building an observatory at such a remote, high-altitude site.