University of Melbourne hydrology professor Dongryeol Ryu and his collaborator Ki-Weon Seo were on a train to visit Ryu’s family when they found something startling. Stopped at a station for technical issues, Seo had pulled out his computer to pass the time with some work when a result popped up in their data that Ryu could hardly believe: It suggested a “remarkable” amount of Earth’s water stored on land had been depleted.

“At first we thought, ‘That’s an error in the model,’” Ryu said.

After a year of checking, they determined it wasn’t.