Here’s something to remember as we observe Earth Day today. There’s just one. “At present, there is no demonstrated alternative to maintaining the viability of Earth,” said Joel E. Cohen, a populations professor at The Rockefeller University in New York City. “Despite its mysteries and hazards, Earth remains the only known home that can sustain life.” Arizona was host to the grandest experiment to re-create a piece of Earth: Biosphere 2. And the experiment failed.