ECOLOGIST Jonathan Pauli used to spend a lot of time keeping track of animals over winter – often across cold, harsh landscapes that seemed inhospitable to life. It always surprised him that as soon as the weather got warmer in early spring, insects would pop up. “Snow fleas would emerge from underneath the snow,” Pauli recalls. Where, he wondered, had they been hiding? Eventually, he discovered some old scientific papers from the 1940s and 1960s. They revealed a secret world that Pauli, a researcher at the University of Wisconsin-Madison, has been studying ever since: a hidden ecosystem under the snow.