In Brazil, a large sink hole is opening up in the city of Buriticupu, in the northeastern Amazon. Over a thousand people in a neighborhood on the edge of town are watching their homes slowly fall off a cliff. This type of erosion known locally as voçoroca has triggered a state of emergency. Matthias Vanmaercke knows the phenomenon by a different name — it’s called an urban gully. He’s a professor of geography at University of Leuven in Belgium and says this is happening in and around urban areas across the globe.